Preserve unpacked GenericAlias state during substitution - #9467

Summary

Preserve the unpacked (starred) flag when substituting parameters of a GenericAlias. CPython's ga_getitem copies this flag onto the newly created alias; RustPython was always resetting it to false.

For example, substituting into an unpacked *tuple[T] should retain the unpacked alias and its __unpacked__ value rather than returning an ordinary tuple[int] alias.

• CPython source: https://github.com/python/cpython/blob/d220409a13b77de7a6c30abe16ea90db01528edc/Objects/genericaliasobject.c#L595-L617
• Synchronize the complete Lib/test/test_typing.py using scripts/update_lib migrate from that pinned CPython source. The result is byte-for-byte identical upstream, retiring the variadic substitution expected-failure marker and its otherwise-unused import.

Verification

• Scoped VM cargo check and Clippy passed.
• A fresh CLI build in a dedicated target directory passed.
• Six unchanged CPython tests passed on both CPython and the candidate: the four GenericAliasSubstitutionTests methods plus test_genericalias.BaseTest.test_unpack and test_iter_creates_starred_tuple. No skips or expected failures in this selection.
• Eleven finite ordinary/unpacked/nested/variadic substitution cases produced identical full JSON transcripts on both interpreters, including representation, origin, arguments, parameters and unpacked state.
• Normal commit hooks passed, including rustfmt, Ruff and cspell.

The full workspace/C-API suite and complete test_typing module were not run locally. This draft awaits normal CI for broader coverage.

Part of the Python 3.15 upgrade split from #8954. No new snippets or Rust tests of Python behavior are added.