Compatibility: use pytest instead of nose

[antirotor]

Problem

Using long unmaintained and abandoned nose for running tests started finally breaking with python 3.11 and higher. This PR is fixing two issues:

• invalid escape sequence in iscompatible.py breaking on python 3.12 because warnings became SyntaxErrors
• switch fromnose to pytest (some cleanup still pending and dependency handling delegated to uv
• switch to uv

[antirotor]

There are also two tests failing (one silently), not related most likely to switch to pytest:

tests/test_plugin::test_argumentless_explitic_plugin (seems like for Instance plugin, the check for arguments doesn't work?
tests/test_events.py::test_plugin_failed_event counting failures doesn't work

[mottosso]

I've added an alternative solution here, that doesn't require a change of framework with more (many more!) dependencies.

• Update run_testsuite.py to support Python 3.10+ #409

pytest not only pulls in tons of dependencies, but some of those dependencies are compiled for the platform as well, making it difficult to vendor.

Have a look and see if it fits.

[antirotor]

I've separated #410 from this as it is the real blocker.