Update pip to 19.3.1

[pyup-bot]

This PR updates pip from 19.3 to 19.3.1.

Changelog

19.3.1

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

Features
--------

- Document Python 3.8 support. (`7219 <https://github.com/pypa/pip/issues/7219>`_)

Bug Fixes
---------

- Fix bug that prevented installation of PEP 517 packages without ``setup.py``. (`6606 <https://github.com/pypa/pip/issues/6606>`_)

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 #90