Adopt Python v3.13

[pp-mo]

Closes #506

Modelled on #461, frankly.

It's too soon to drop Python 3.10.
Also may not work, given problems encountered in SciTools/iris#6195

But it's certainly worth a go.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.41%. Comparing base (578fcdf) to head (5c73774).
Report is 30 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #512   +/-   ##
=======================================
  Coverage   58.41%   58.41%           
=======================================
  Files          62       62           
  Lines        6435     6435           
  Branches     1150     1150           
=======================================
  Hits         3759     3759           
  Misses       2385     2385           
  Partials      291      291           

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[pp-mo]

Frankly I didn't expect that to work !

But now it seems all good
-- unless I misunderstood something like test_python_versions ??
Could you maybe cast an eye over this, @trexfeathers ?

[pp-mo]

Thanks @trexfeathers !