Creo que tengo un problema con mi sistema Mac OS. Todo lo que trato de instalar usando pip me sale los mismos errores una y otra vez, he pegado solo las líneas que muestran un error para no saturar este hilo:

Collecting murmurhash3
  Using cached https://files.pythonhosted.org/packages/b5/f4/1f9c4851667a2541bd151b8d9efef707495816274fada365fa6a31085a32/murmurhash3-2.3.5.tar.gz
Building wheels for collected packages: murmurhash3
  Running setup.py bdist_wheel for murmurhash3 ... error
  Complete output from command /usr/local/opt/python/bin/python3.7 -u -c "import setuptools, tokenize;__file__='/private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-install-_4hr8w03/murmurhash3/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-wheel-k28wu_2e --python-tag cp37:
  running bdist_wheel
  running build
  running build_ext
  building 'mmh3' extension
  creating build
  creating build/temp.macosx-10.14-x86_64-3.7
  clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -I/usr/include -I/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m -c mmh3module.cpp -o build/temp.macosx-10.14-x86_64-3.7/mmh3module.o
  In file included from mmh3module.cpp:3:
  /usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/Python.h:14:2: error: "Something's broken.  UCHAR_MAX should be defined in limits.h."
  #error "Something's broken.  UCHAR_MAX should be defined in limits.h."
   ^
  /usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/Python.h:18:2: error: "Python's source code assumes C's unsigned char is an 8-bit type."
  #error "Python's source code assumes C's unsigned char is an 8-bit type."
   ^
  mmh3module.cpp:99:5: error: use of undeclared identifier 'memcpy'; did you mean 'wmemcpy'?
      memcpy(bytes, result, 16);
      ^~~~~~
      wmemcpy
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/wchar.h:152:10: note: 'wmemcpy' declared here
  wchar_t *wmemcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
           ^
  mmh3module.cpp:99:12: error: cannot initialize a parameter of type 'wchar_t *' with an lvalue of type 'char [16]'
      memcpy(bytes, result, 16);
             ^~~~~
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/wchar.h:152:38: note: passing argument to parameter here
  wchar_t *wmemcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
                                       ^
  4 errors generated.
  error: command 'clang' failed with exit status 1

  ----------------------------------------
  Failed building wheel for murmurhash3
  Running setup.py clean for murmurhash3
Failed to build murmurhash3
Installing collected packages: murmurhash3
  Running setup.py install for murmurhash3 ... error
    Complete output from command /usr/local/opt/python/bin/python3.7 -u -c "import setuptools, tokenize;__file__='/private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-install-_4hr8w03/murmurhash3/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-record-7a15nstl/install-record.txt --single-version-externally-managed --compile:
    running install
    running build
    running build_ext
    building 'mmh3' extension
    creating build
    creating build/temp.macosx-10.14-x86_64-3.7
    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -I/usr/include -I/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m -c mmh3module.cpp -o build/temp.macosx-10.14-x86_64-3.7/mmh3module.o
    In file included from mmh3module.cpp:3:
    /usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/Python.h:14:2: error: "Something's broken.  UCHAR_MAX should be defined in limits.h."
    #error "Something's broken.  UCHAR_MAX should be defined in limits.h."
     ^
    /usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/include/python3.7m/Python.h:18:2: error: "Python's source code assumes C's unsigned char is an 8-bit type."
    #error "Python's source code assumes C's unsigned char is an 8-bit type."
     ^
    mmh3module.cpp:99:5: error: use of undeclared identifier 'memcpy'; did you mean 'wmemcpy'?
        memcpy(bytes, result, 16);
        ^~~~~~
        wmemcpy
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/wchar.h:152:10: note: 'wmemcpy' declared here
    wchar_t *wmemcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
             ^
    mmh3module.cpp:99:12: error: cannot initialize a parameter of type 'wchar_t *' with an lvalue of type 'char [16]'
        memcpy(bytes, result, 16);
               ^~~~~
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/wchar.h:152:38: note: passing argument to parameter here
    wchar_t *wmemcpy(wchar_t * __restrict, const wchar_t * __restrict, size_t);
                                         ^
    4 errors generated.
    error: command 'clang' failed with exit status 1

    ----------------------------------------
Command "/usr/local/opt/python/bin/python3.7 -u -c "import setuptools, tokenize;__file__='/private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-install-_4hr8w03/murmurhash3/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-record-7a15nstl/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /private/var/folders/15/_mc1281d6kn_z8my6cz6dj740000gq/T/pip-install-_4hr8w03/murmurhash3/

Por otro lado, cuando intento instalarlxml orequests todo funciona muy bien. ¿Alguna idea sobre dónde me he equivocado?