Prior to pip 21.1, pip install --editable --target fails because it
results in wrong parameters being passed to setup.py develop by pip.

Prior to setuptools 45.2.0, both pip install --editable --target and
pip install --editable --prefix fail because the temporary install
directory used internally by pip is not added to PYTHONPATH prior
to pip calling setup.py develop. In later version setuptools emits a
warning instead of an error.

Temporarily override PYTHONUSERBASE to point to the target directory,
so as to emulate --prefix=<dir> with PYTHONUSERBASE=<dir> and --user.

This is needed for Python2 because pip 21.1 and setuptools 45.2.0 are
both Python3 only.