guix: build all Python packages with --single-version-externally-managed.

This requires setuptools to be installed together with python, which is
the case for Python 3 anyway and which we do for our build of Python 2
(see last commit).

* guix/build/python-build-system.scm (install): Add
  "--single-version-externally-managed" and "--root=/" to params to be
  passed to call-setuppy. Remove thus needless manipulation of
  PYTHONPATH. Remove now unused argument "inputs".