[CI/MAINT]: Tidy CI files and uncompatible dependencies versions

[echedey-ls]

• Removed usual pip requirements that were not really necessary to specify the versions to tests against (a maintenance burden, I did not think about making the install requirements step optional at first)
• Kept & updated minimal versions to use exact version matching (better than ~=)
• Updated the pyproject.toml dependencies versions to match what really is compatible after recent changes and on the lowest supported version.
• Pinned docs dependencies to minor versions to avoid unexpected changes in the future, now that RTD is finally set up.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[echedey-ls]

Merge after #13

[adriesse]

@echedey-ls Could you include the tests in the distribution package while you're at it?

[echedey-ls]

@adriesse do you mean shipping them in the wheel too? They already are in the source distribution.

EDIT: for reference, see these files in https://pypi.org/project/pvpltools/#files

[echedey-ls]

@echedey-ls Could you include the tests in the distribution package while you're at it?

I've created #16 for that, so you can decide on it and atomically revert that change if you want in the future.