Remove InstallRequirement source_dir parameter

Update docs conventions based on recent experience

Nicer Azure Pipelines setup

This wasn't actually ever used, since the value was always '$(useVenv)'.

source_dir is only passed to the InstallRequirement constructor
in the case of editable requirements, and it is built from link,
which is also passed to the same constructor. So we let
InstallRequirement compute source_dir, to remove that burden
from call sites.

Create FUNDING.yml

Significantly improve release version validation

Co-Authored-By: NXavier Fernandez <xav.fernandez@gmail.com>

Azure Pipelines: Test Linux and macOS on Python 3.8

Fix incorrect comment in tests in test_install

New Resolver: Implement equality on candidate classes

Generate legacy metadata in temporary directory

Prevent infinite recursion with pip wheel with $TMPDIR in $PWD

New Resolver: Make sure candidates are prepared after resolution

Add unit tests for pip commands not using cwd

New Resolver: Correctly uninstall existing distribution before installing

Remove the version part from the dist-info directory name

Remove shebang from nonexecutable script

Added type annotations to pip._internal.commands.check

Warn if an invalid URL is passed with --index-url

Fix tabulate col size in case of empty cell

When packaging pip in Fedora, we have realised
that there is a nonexecutable file with a shebang line.

It seems that the primary purpose of this file is to be imported from Python
code and hence the shebang appears to be unnecessary.

Shebangs are hard to handle when doing downstream packaging because it makes
sense for upstream to use `#!/usr/bin/env python` while in the RPM package, we
need to avoid that and use a more specific interpreter. Since the shebang was
unused, I propose to remove it to avoid the problems.

We have found more shebangs but in vendored packages. I have also opened PRs there:
https://github.com/ActiveState/appdirs/pull/144
https://github.com/psf/requests/pull/5410
https://github.com/chardet/chardet/pull/192

x

Add ability to run pip with options for yaml tests

Cache parse_links() by --find-links html page url