Signed-off-by: NPradyun Gedam <pradyunsg@users.noreply.github.com>

Don't notify pypa-dev IRC on build status

Use the new environment files in GitHub Actions

Signed-off-by: NPradyun Gedam <pradyunsg@users.noreply.github.com>

Update message displayed on conflicts post-resolution

Use set to dedup package list in error message

cli: Drop --unstable-feature flag

An additional section is added to instruct the user to self-check
whether pip is installed before proceeding.

The Linux package manager section is moved to the front to encourage
users to try that first before learning about get-pip.py, which tends to
provide seemingly working but subtly broken results there. A paragraph
is added to the section to instruct users to report issues to package
managers instead.

A new section on enturepip is added as an alternative to get-pip.py,
which is almost guarenteed to be available at this point.

Signed-off-by: NPradyun Gedam <pradyunsg@users.noreply.github.com>

Furo provides a fully-fleshed-out right sidebar that provides an in-page
table of contents, that's scrollable and does not interrupt content.

Make sure periods are followed by a space

The strings are *delibrately* reformatted so line continuations occur in
the middle of sentences. This helps me ensure all spaces are added
properly.

Remove unused comes_from argument of parse_requirements

Tie-break requirements with package name

This makes the ordering deterministic to improve debugging and user
experience.

Use packaging.version to check version equality

This is possible because self.make_candidate_evaluator only depends
on:
- the function arguments which are keys to the cache
- self._target_python which never changes during a pip resolution
- self._candidate_prefs which never changes during a pip resolution

On a fresh install, pip install <a package with ~ 100 dependencies>
runs on my machine in:

master (a0e34e9c)
=======================

0m33.058s
0m34.105s
0m32.426s

This commit
===========

0m15.860s
0m16.254s
0m15.910s

pip 20.2.4 - legacy resolver
============================

0m15.145s
0m15.040s
0m15.152s