Initial support for Python 3.15.0a8

[cdce8p]

Some minor changes to be able to compile mypy with mypyc on Python 3.15.0a8.

• Update typing-extensions to >=4.14.0 for Python >=3.15 due to the removal of typing.no_type_check_decorator
  Remove deprecated typing.no_type_check_decorator cpython#133601
  https://docs.python.org/3/library/typing.html#typing.no_type_check_decorator
  https://github.com/python/typing_extensions/blob/4.14.0/CHANGELOG.md
• TypeAliasType.__qualname__ was added in TypeAliasType objects have no __qualname__ cpython#139817
• Some new FutureWarnings for b64_decode
  Base64 decode with altchars still accepts non-altchars cpython#125346
  gh-125346: Fix decoding with non-standard Base64 alphabet cpython#141128

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Thanks!

[cdce8p]

@hauntsaninja Is this something worth cherry-picking for the 1.20.2? It would allow users to compile mypy with mypyc themselves for 3.15.0a8 if they want to.

[hauntsaninja]

Seems like a good idea, thanks for suggesting it!