gh-106246: Allow the use of unions as match patterns

[randolf-scholz]

Fixes #106246

Modified version of #118525 that supports using subpatterns in unions.

• Issue: [match-case] Allow matching Union types #106246

added tests:

1. test_patma_union_type
2. test_patma_union_no_match
3. test_union_type_positional_subpattern
4. test_union_type_keyword_subpattern
5. test_patma_union_arg
6. test_patma_union_kwarg
7. test_patma_union_arg_no_match
8. test_patma_union_kwarg_no_match
9. test_union_type_match_second_member
10. test_generic_type
11. test_legacy_generic_type
12. test_class_or_union_not_specialform
13. test_legacy_union_type
14. test_expanded_union_mirrors_isinstance_success
15. test_expanded_union_mirrors_isinstance_failure
16. test_union_type_mirrors_isinstance_success
17. test_union_type_mirrors_isinstance_failure
18. test_generic_union_type

[tmke8]

Thanks for working on this!

[sobolevn]

Too late for 3.13 :(
This will go into 3.14

[gvanrossum]

While you have some time, I recommend adding some great docs for the new feature as well.