Add 3.13 to basepython

[sentrivana]

...once available.

See https://tox.wiki/en/latest/config.html#python-options

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.27%. Comparing base (d0eca65) to head (c3509bb).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3589   +/-   ##
=======================================
  Coverage   84.27%   84.27%           
=======================================
  Files         133      133           
  Lines       14068    14068           
  Branches     2966     2966           
=======================================
  Hits        11856    11856           
  Misses       1471     1471           
  Partials      741      741           

see 3 files with indirect coverage changes