depends_on :python sets PYTHONPATH internally

This applies only to non-brewed python:
The user will still have to set his PYTHONPATH
in order to import python modules in an
external python interpreter. However, a
warning will be displayed, now.

Further, during the build, the PYTHONPATH
will be set to Homebrew's global site-packages
so that software (like PyQt and others) that
need to `import sipconfig` in python can
find the brewed sip and its python module.
MacVim: Link python statically