Update pip to 21.2

[pyup-bot]

This PR updates pip from 21.1.2 to 21.2.

Changelog

21.1.3

===================

Bug Fixes
---------

- Remove unused optional ``tornado`` import in vendored ``tenacity`` to prevent old versions of Tornado from breaking pip. (`10020 <https://github.com/pypa/pip/issues/10020>`_)
- Require ``setup.cfg``-only projects to be built via PEP 517, by requiring an explicit dependency on setuptools declared in pyproject.toml. (`10031 <https://github.com/pypa/pip/issues/10031>`_)

Links

• PyPI: https://pypi.org/project/pip
• Changelog: https://pyup.io/changelogs/pip/
• Homepage: https://pip.pypa.io/

[pyup-bot]

Closing this in favor of #264