Update pip to 19.1.1

[pyup-bot]

This PR updates pip from 19.1 to 19.1.1.

Changelog

19.1.1

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

Features
--------

- Restore ``pyproject.toml`` handling to how it was with pip 19.0.3 to prevent
the need to add ``--no-use-pep517`` when installing in editable mode. (`6434 <https://github.com/pypa/pip/issues/6434>`_)

Bug Fixes
---------

- Fix a regression that caused `` to be quoted in pypiserver links.
This interfered with parsing the revision string from VCS urls. (`6440 <https://github.com/pypa/pip/issues/6440>`_)

Links

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