Update pip to 19.0.3

[pyup-bot]

This PR updates pip from 19.0.2 to 19.0.3.

Changelog

19.0.3

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

Bug Fixes
---------

- Fix an ``IndexError`` crash when a legacy build of a wheel fails. (`6252 <https://github.com/pypa/pip/issues/6252>`_)
- Fix a regression introduced in 19.0.2 where the filename in a RECORD file
of an installed file would not be updated when installing a wheel. (`6266 <https://github.com/pypa/pip/issues/6266>`_)

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 #35