124: pip dependency resolver changes

pip is the package installer for Python. 

Often, when you run pip, especially the first time in a new virtual environment, you will see something like:

WARNING: You are using pip version 20.1.1; however, version 20.2 is available.

 You should consider upgrading via the 'python -m pip install --upgrade pip' command.

And you should. Because 20.2 has a new dependency resolver.

Get in the habit, until October, of replacing pip install with pip install --use-feature=2020-resolver.

 This flag is new in the 20.2 release.

This new pip dependency resolver is the result of a lot of work.

 Five of the people involved with this work are joining the show today: Bernard Tyers, Nicole Harris, Paul Moore, Pradyun Gedam, and Tzu-ping Chung. 

We talk about: 

• pip dependency resolver changes
• user experience research and testing
• crafting good error messages
• efforts to improve the test suite
• testing pip with pytest
• some of the difficulties with testing pip
• working with a team on a large project
• working with a large code base
• bringing new developers into a large project

Special Guests: Bernard Tyers, Nicole Harris, Paul Moore, Pradyun Gedam, and Tzu-ping Chung.

Links:

• Changelog — pip 20.2 documentation — Including --use-feature=2020-resolver
• pypa/pip: The Python package installer — github repo
• testing pip - documentation
• pip - The Python Package Installer — pip 20.2 documentation
• Changes to the pip dependency resolver in 20.2 — Changes to the pip dependency resolver in 20.2