Use `pip` for installing Pytest in containers

[brockdyer03]

Current

Currently the containers used in CI testing install every package from apt or dnf, which is probably fine for most dependencies since they should ideally be compatible with what some users would use.

This has become a problem for the migration to Pytest (#5881), which generally relies on modern Pytest constructs. There have been several spots in #5894 where there is extra logic specifically for handling various Pytest versions.

That logic may be simple on the surface, but I personally have spent far, far more time and energy than I wanted to just trying to figure out which features I can use and which I can't use.

Proposed

It would be much easier to just install pytest, pytest-cov, and pytest-order with pip in the containers so that I can actually know that the version is consistent. This wouldn't necessarily affect the generality of the tests since the other Python packages are still installed via apt or dnf, and those are what the few users who are, for one reason or another, unable to use a package manager, actually care about.

[prckent]

What modern pytest constructs are needed? What are the minimum versions?

[brockdyer03]

There aren't any things that are strictly necessary that I've found so far, but the problem is more that it's very difficult to know what I can use. I might've just gotten lucky for now, but in the future we can't rely on luck.

Some things that would be very nice to have are the ability to use pytest.toml instead of the likely-to-be-deprecated [tool.pytest.ini_options]. There are also possibly conflicts between pytest-cov's changes to subprocess measurement that occurred when they updated from version 6->7. Additionally, it will mean that we can actually use the CentOS containers since they are currently limited by dnf.

The minimum versions are listed in the pyproject.toml file. Those are very recent, and they are likely to be the least work to maintain in the future.