Support PEP 696 - Type Defaults for Type Parameters

[rchen152]

PEP accepted for 3.13: https://peps.python.org/pep-0696/.

[h-joo]

Hi, thank you for your report. Google is shifting its effort to a different approach for type checking apart from pytype, and we're not planning to put in any effort in the near future in pytype, so we'll close this issue. Please see the announcement here.