Summary

• Run most tests in a single Python instance, initialized at start and shutdown at the end
• Improve uv CI setup
• Fix python test runner by essentially replacing it with a call to pytest.main
• Improve the PythonDLL exception
• Run tests that require reinitialization of the PythonEngine only on .NET Framework/Mono, where we still have a working formatter for the stashing

TODO:

• Remove PythonEngine.Initialize() and .Shutdown() from all fixtures where it's not strictly necessary
• Ensure that all fixtures reset the environment (e.g. by adjusting the base type providers and codecs back to the default)
• Move the remaining fixtures into a separate project that runs all actual tests in individual processes (not needed, we can run those tests on Mono)
• Reenable all disabled tests (all but the low-level ones)
• Fix the domain reload tests (or ditch them)