Replace python2 deprecation with a badge of supported python versions

The python world has (mostly) moved on from Python 2. Anyone not already aware of the py2->py3 migration is probably new to the ecosystem and started on Python 3.

Additionally, it's convenient to see at a glance what versions of Python are supported by the current release. This pulls from PyPI versions, so will not immediately match `main` (we can probably change this to match `main` if preferred).

So by doing this it's both more useful going forward, and also lets us drop the explicit notice about dropping Python 2 support.

README.rst

@@ -3,9 +3,15 @@ pip - The Python Package Installer
 
 .. image:: https://img.shields.io/pypi/v/pip.svg
    :target: https://pypi.org/project/pip/
+   :alt: PyPI
+
+.. image:: https://img.shields.io/pypi/pyversions/pip
+   :target: https://pypi.org/project/pip
+   :alt: PyPI - Python Version
 
 .. image:: https://readthedocs.org/projects/pip/badge/?version=latest
    :target: https://pip.pypa.io/en/latest
+   :alt: Documentation
 
 pip is the `package installer`_ for Python. You can use pip to install packages from the `Python Package Index`_ and other indexes.
 
@@ -19,8 +25,6 @@ We release updates regularly, with a new version every 3 months. Find more detai
 * `Release notes`_
 * `Release process`_
 
-**Note**: pip 21.0, in January 2021, removed Python 2 support, per pip's `Python 2 support policy`_. Please migrate to Python 3.
-
 If you find bugs, need help, or want to talk to the developers, please use our mailing lists or chat rooms:
 
 * `Issue tracking`_
@@ -47,7 +51,6 @@ rooms, and mailing lists is expected to follow the `PSF Code of Conduct`_.
 .. _Release process: https://pip.pypa.io/en/latest/development/release-process/
 .. _GitHub page: https://github.com/pypa/pip
 .. _Development documentation: https://pip.pypa.io/en/latest/development
-.. _Python 2 support policy: https://pip.pypa.io/en/latest/development/release-process/#python-2-support
 .. _Issue tracking: https://github.com/pypa/pip/issues
 .. _Discourse channel: https://discuss.python.org/c/packaging
 .. _User IRC: https://kiwiirc.com/nextclient/#ircs://irc.libera.chat:+6697/pypa