WARNING: fetchcode's vcs will not work on this commit.
This is result of separating pip's code from changes made in scope of
this repository.

This should make it easier to track potentially replicated issues from
pip when taking their vcs pkg.

It also made cleaning up easier, due to some maintenance activities done
in pip:
- dropping Python 2 & 3.5 support
  pypa/pip#9189
- modernized code after above - partially done, tracked in:
  pypa/pip#8802
- added py3.9 support
- updated vendored libraries (e.g. fixing CVE-2021-28363)
  multiple PRs

pip._internal.vcs (and related code) changes:
- Fetch resources that are missing locally:
  pypa/pip#8817
- Improve SVN version parser (Windows)
  pypa/pip#8665
- Always close stderr after subprocess completion:
  pypa/pip#9156
- Remove vcs export feature:
  pypa/pip#9713
- Remove support for git+ssh@ scheme in favour of git+ssh://
  pypa/pip#9436
- Security fix in git tags parsing (CVE-2021-3572):
  pypa/pip#9827
- Reimplement Git version parsing:
  pypa/pip#10117

In next commits, most of pip's internals will be removed from fetchcode,
leaving only vcs module with supporting code (like utils functions,
tests (which will be added & submitted with this change))

This will allow for changes such as ability to add return codes
(probably via futures) from long running downloads and other features.

Switching to having own vcs module might also be a good call due to
pip._internal.vcs close integration with pip's cli in vcs module (some
pip code has been commented out in commit mentioned below)

While generally copy-pasting code without history is bad idea, this
commit follows precedence set in this repo by:
8046215
with exception that all changes to pip's code will be submitted as separate
commits.

It has been agreed with @pombredanne & @TG1999 that history from pip
will be rebased on fetchcode by @pombredanne (thanks!). It will be done
only for the files that are of concern for fetchcode to limit noise in
git history.

I'm leaving this commit without SoB intentionally, as this is not my
work, but that of the many pip's authors:
https://github.com/pypa/pip/blob/21.2.4/AUTHORS.txt
License of pip: MIT (https://pypi.org/project/pip/)