Update annotation parsing to work with PEP 649 in Python 3.14

[jcrist]

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It looks like accessing __annotations__ is likely to change in Python ~3.13~ 3.14.

References:

• https://peps.python.org/pep-0649/
• https://github.com/beartype/beartype/discussions/335

AFAICT none of this has actually landed in the cpython git repo yet - if/once it does we can start evaluating what changes are needed here.