Fix accidental changes to PYTHONPATH in os.environ

[xavth]

When generating an environment dict for subprocess calls to pip, os.environ was accidentally modified despite efforts to copy it and modify only the copy, as copy.copy(os.environ) is not enough.

Quick proof in a python shell:

>>> import os
>>> import copy
>>> env = copy.copy(os.environ)
>>> 'abc' in os.environ
False
>>> env['abc'] = 'xyz'
>>> 'abc' in os.environ
True

[perrinjerome]

That's true. pylint has a linter rule for this https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/shallow-copy-environ.html

BTW, in the discussions from https://bugs.python.org/issue15373 this nice approach is discussed:

    exit_code = subprocess.call(list(args), env=dict(os.environ, PYTHONPATH=python_path))