We should be able to install via pip without checking the python3 on the PATH

[adamnovak]

Jonas managed to produce this:

    running build_ext
    cmake /private/var/folders/zy/0_dym7ln2b9bc0ywr4by65jr0000gn/T/pip-install-e9mb__6t/bdsg -DRUN_DOXYGEN=OFF -DPYTHON_EXECUTABLE=/usr/local/opt/python/bin/python3.7 -DCMAKE_LIBRARY_OUTPUT_DIRECTORY=/private/var/folders/zy/0_dym7ln2b9bc0ywr4by65jr0000gn/T/pip-install-e9mb__6t/bdsg/build/lib.macosx-10.14-x86_64-3.7 -DCMAKE_BUILD_TYPE=Release
...
    CMake Error at CMakeLists.txt:182 (message):
      Python version mismatch: CMake wants to build for Python 3.7.6 at
      /usr/local/opt/python/bin/python3.7 but `python3` is Python 3.5.3 at
      /usr/local/bin/python3.  You will not be able to import the module in the
      current Python! To use the version CMake selected, run the build in a
      virtualenv with that Python version activated.  To use the version on your
      PATH, restart the build with -DPYTHON_EXECUTABLE=/usr/local/bin/python3 on
      the command line.

If -DPYTHON_EXECUTABLE is explicitly set, or maybe if some new CMake option is set, we should let setup.py bypass that check, since under setup.py we are pretty sure we have the right python.