Misc fixes

[skirpichev]

• drop workaround for CPython 3.13, see python/cpython#112115
• port more functions/methods to METH_FASTCALL convention
• simplify _mpmath_normalize/_mpmath_create
• update copyright

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (95a4ed2) 85.51% compared to head (2132a2d) 85.55%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #466 +/- ## ========================================== + Coverage 85.51% 85.55% +0.03% ========================================== Files 49 49 Lines 11691 11678 -13 Branches 2211 2209 -2 ========================================== - Hits 9998 9991 -7 + Misses 1693 1687 -6 ```

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[casevh]

Thanks for the cleanup.