Add an option to install the oldest available versions of dependencies

[azmeuk]

What's the problem this feature will solve?

Often dependencies versions are defined with a minimum and a maximum, with foobarlib>=1.0.0,<2 or foobarlib=^1.0.0. However when tox or GHA (or other similar tools) install a project to run unit tests, pip installs the most recent possible version (if we ignore version incompatibility issues).
Generally, minimum versions of dependencies are not tested, and pip provide no easy way to install foobar>=1.0.0 so a CI pipeline can check if that version is really still supported.

Describe the solution you'd like

The simplest thing I can think of from a user perspective would be a --minimum-versions option to pip install that would install the oldest version possible.

Alternative Solutions

The workaround is to run sed in the CI to replace >=x.y.z by ==x.y.z and ^x.y.z by ==x.y.z in setup.cfg/pyproject.toml files.
This is not ideal because the regex can easily break, sed might have different behavior depending on the platform, that workaround is cumbersome to run locally.

Additional context

As an example, ihatemoney implements the sed workaround.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

Duplicate of #8085