Support for pytest

[fsuba]

Currently there are two frameworks supported in tmt tests specification:

framework: shell
framework: beakerlib

Wouldn't it make sense to also include pytest?
Also, plan prepare step accepts package, which has to be rpm. What if we could also specify pypi packages to be installed?
When pytest framework is selected, there could be python venv created for that plan.
Related to #1922

[happz]

There were some voices looking for this kind of support, and there's an issue for leveling the playfield to allow other players besides shell and beakerlib: #1305

[thrix]

Btw, people now run pytest tests, see convert2rhel, e.g. https://github.com/oamg/convert2rhel/tree/main/tests/integration/tier0/assessment-report

Of course, tighter integration could be nicer, usability wise, but I believe you can now run pytest tests via tmt ...

Patches welcome ;) tmt is open-source.