pipenv: fix dependencies

- setup_requires belong in nativeBuildInputs
- requests is only for Python 2. We offer only a Python 3 version of
pipenv
- setting PYTHONPATH is not needed because the magical sed expression
injects the dependencies in the executables. Otherwise, we would use
NIX_PYTHONPATH.
- PIP_IGNORE_INSTALLED was needed because of PYTHONPATH, but since we do
not set that anymore we can remove.