mypy 1.12 prep

[mr-c]

• prepare for mypy 1.12
• gh-actions: loosen python 3.13 version specification

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.07%. Comparing base (c12f1ed) to head (f3cf3ad). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
schema_salad/avro/schema.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #876 +/- ## ======================================= Coverage 83.07% 83.07% ======================================= Files 22 22 Lines 4886 4886 Branches 1326 1326 ======================================= Hits 4059 4059 Misses 548 548 Partials 279 279 ```

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