Misc fixes

[skirpichev]

• drop workaround for CPython 3.13, see gh-112026: Restore removed private C API 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 ✅

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

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Additional details and impacted files

@@            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.