Update marsh package pin in environment.yml

[rickecon]

• make format and make documentation has been run. (You may also want to run make test.)

This PR:

• Updates environment.yml and setup.py to pin to version marshmallow<4.0.0. This is a temporary fix due to an issue with higher versions of marshmallow interacting with paramtools.

cc: @jdebacker

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.29%. Comparing base (f034090) to head (bb389d3).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   59.29%   59.29%           
=======================================
  Files          12       12           
  Lines         425      425           
=======================================
  Hits          252      252           
  Misses        173      173           

Flag	Coverage Δ
unittests	59.29% <100.00%> (ø)

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[jdebacker]

LGTM. Thanks @rickecon