Update annotation parsing to work with PEP 649 in Python 3.13

[Jim Crist-Harif]

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

References:

• https://peps.python.org/pep-0649/
• beartype/beartype#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.