Add Python 3.13 alpha to CI

[Gallaecio]

Current blockers:

• zstandard (directly and via mitmproxy)
• pywin32
• Pillow
• typing-extensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.92%. Comparing base (180bc9b) to head (04bc1e6).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6166      +/-   ##
==========================================
- Coverage   85.00%   84.92%   -0.08%     
==========================================
  Files         161      162       +1     
  Lines       11962    12112     +150     
  Branches     1872     1839      -33     
==========================================
+ Hits        10168    10286     +118     
- Misses       1512     1538      +26     
- Partials      282      288       +6     

see 9 files with indirect coverage changes