Add ability to install oldest version of package

[adamjstewart]

What's the problem this feature will solve?

In my CI, I currently use pip to install all of my dependencies for testing. By default, pip will install the newest version of each package, which helps me check for changes to my dependencies. However, I would also like to be able to tell pip to install the oldest supported version of each dependency so I can make sure I don't add new features without bumping the version requirement.

Describe the solution you'd like

If I have a requirements.txt like:

black>=20
flake8~=3.7
isort

I would like to be able to run pip install --oldest -r requirements.txt and have pip install the oldest version of each package that it knows about that satisfies the requirements. The idea would be to run all of my tests with a job matrix that uses both --oldest and --newest (or no flag) to make sure things work.

Alternative Solutions

For now, the only thing I can think of is to use sed to replace >= and ~= with ==. However, this doesn't work for packages without a minimum requirement, or for packages where the version is only partially specified.

[uranusjr]

#8085