Drop support for Python 3.9

[binaryDiv]

This PR removes support for Python 3.9, raising the minimum Python version to 3.10, in order to allow for more modern typing (union types with X | Y), and modernizes the code base.

Additionally, this allowed us to remove a rather hacky workaround that was needed for Python < 3.10, where dataclasses did not support kw_only yet. This also removes the need for coverage-conditional-plugin.

I also updated the testing dependencies.

This qualifies as a breaking change in terms of Python version compatibility. There are no breaking API changes, however.

[JGaukrogers]

I see no changes in the file validataclass_mixin_test.py, but there is the method test_create_with_defaults_invalid_required_fields, which contains the comment # The exact exception message changed between Python versions 3.9 and 3.10. Now I am wondering if there needs to be adapted something, or if we can leave it as it is.

[binaryDiv]

@JGaukrogers

I see no changes in the file validataclass_mixin_test.py, but there is the method test_create_with_defaults_invalid_required_fields, which contains the comment # The exact exception message changed between Python versions 3.9 and 3.10. Now I am wondering if there needs to be adapted something, or if we can leave it as it is.

Well spotted! :D

I was thinking about changing that too, but I don't think it's really worth changing. For one thing, just partially matching the error message is sufficient I think, and also it's a test for a deprecated function that will be removed soon, anyway.

(Actually, I think it's probably a good time to remove it in the upcoming release, it's been deprecated for a while and I don't think it's still used anywhere in our projects... But that's for another PR ^^)