Remove pin of marshmallow

[jdebacker]

This PR updates environment.yml to remove the pin to marshmallow<3.22, which is not necessary with ParamTools>=0.18.3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (0dff0c3) to head (8d8d518).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2799   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          13       13           
  Lines        2614     2614           
=======================================
  Hits         2599     2599           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <ø> (ø)

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