Update pip to 19.2.3

[pyup-bot]

This PR updates pip from 19.2.1 to 19.2.3.

Changelog

19.2.2

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

Bug Fixes
---------

- Fix handling of tokens (single part credentials) in URLs. (`6795 <https://github.com/pypa/pip/issues/6795>`_)
- Fix a regression that caused ``~`` expansion not to occur in ``--find-links``
paths. (`6804 <https://github.com/pypa/pip/issues/6804>`_)

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 #243