Remove site file usage

[jaraco]

In #1998 and pypa/virtualenv#1638 and other places, we learned that:

• virtualenv will not be superseded by venv.
• virtualenv 20 still (and more aggressively) uses a site.py file to hack into Python.
• As a result, virtualenv is the only package allowed to use site.py and setuptools needs to drop support for eggs on PYTHONPATH taking precedence over other packages on sys.path or any other features that rely on site.py.

@gaborbernat Does that sound about right?

[gaborbernat]

Sounds about right for python 2. On python 3 virtualenv does not use site.py at all. I'd say site.py is reserved for OS usage though, consider using the site customize file which was designed for such usage.

[jaraco]

I see. That's a little more awkward then. That means that setuptools on Python 2.7 is incompatible with virtualenv 20. So users are kinda screwed anyway, since setuptools doesn't declare a dependency on virtualenv and can't really declare an incompatibility.

That means the only way to support virtualenv 20 is to remove this functionality from setuptools 44.x. I'm a little reluctant to do that as it doesn't give a lot of wiggle room to navigate between Python 2 compatibility and virtualenv compatibility. That is, if setuptools removes this functionality in setuptools 44.2, 44.1.1 becomes the last supported version with this functionality (for Python 2). I guess that's okay if no one misses it.

Okay, I think I have a plan:

• Remove this functionality on Setuptools N+1 (Python 3 only).
• Work through any issues reported by users.
• Assuming all issues are relatively workable, backport the removal to Setuptools 44.x.