Add release notes for 0.13.1

[dstansby]

Once #591 is merged, it would be good to do a release with Python 3.13 support.

Might be good to get #560 in too?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (3b86d9c) to head (bd836a6).
Report is 65 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #592   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files          59       59           
  Lines        2332     2332           
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  Hits         2330     2330           
  Misses          2        2