- Separate build vs. install: the Python bindings will now be built
  during "make all", and installed during "make install" (vs. being
  built and installed during "make install").
  - Fix regression introduced in 9cd0aa4f where the Python bindings
    were not actually installed in $prefix.
- Update from Python distutils to setuptools.  We still use setup.py
  instead of the newer setup.cfg or pyproject.toml, but that can be
  upgraded later.
  - Suppress "setup.py" deprecated warnings during the installation.
- Fix keywords listing in the Python package
- Setup dependencies properly so that the Python bindings will
  properly re-build if any of its dependent header files change.
- Use a workaround to make Cython work properly in VPATH builds.
- Update "make uninstall" for the Python bindings to be a bit more
  robust.
- Make "make clean" remove the generated constants files.
- Removed some stale kruft from setup.py; updated to use setuptools
  instead of disttools.
- Add bindings/python/requirements.txt

Signed-off-by: Jeff Squyres <jeff@squyres.com>