Update packaging to 20.5

[pyup-bot]

This PR updates packaging from 20.4 to 20.5.

Changelog

20.5

~~~~~~~~~~~~~~~~~

* Officially support Python 3.9 (:issue:`343`)
* Deprecate the ``LegacyVersion`` and ``LegacySpecifier`` classes (:issue:`321`)
* Handle ``OSError`` on non-dynamic executables when attempting to resolve
the glibc version string.

Links

• PyPI: https://pypi.org/project/packaging
• Changelog: https://pyup.io/changelogs/packaging/
• Repo: https://github.com/pypa/packaging

[duckinator]

bors r+

[bors]

Build succeeded:

• Linux container:python:3.7-slim
• Linux container:python:3.6-slim
• Zipapp_bootstrap container:python:3.8-slim
• macOS PYTHON:3.6.8
• FreeBSD PYTHON:3.6
• FreeBSD PYTHON:3.7
• Lint
• Linux container:python:3.8-slim
• macOS PYTHON:3.7.2
• Zipapp_bootstrap container:python:3.7-slim