Update obsentry.py to remove deprecated allow_mutation=False

[lewisblake]

Tried setting frozen=True but this breaks 25 tests in tests/aeroval and tests/cams2_83, so for now we keep it mutable

Change Summary

Title

Related issue number

NA

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.85%. Comparing base (114ecdd) to head (665a27c).
Report is 18 commits behind head on main-dev.

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1367   +/-   ##
=========================================
  Coverage     78.85%   78.85%           
=========================================
  Files           136      136           
  Lines         20849    20849           
=========================================
  Hits          16441    16441           
  Misses         4408     4408           

Flag	Coverage Δ
unittests	78.85% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[thorbjoernl]

LGTM 👍