Make frozendict picklable

[tmi]

Replaces frozendict implementation by existing stdlib thing which behaves more or less the same, but coops with pickle

I've tested like d = frozendict({'a': 4}) and d['a'] == 4 and d['a'] = 5 # raises and del d['a'] # raises and d['b'] = 7 # raises. Are there any other cases of interest?

(ftr I put no changelog here, this should have no outwardly visible effect)

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.00%. Comparing base (91b104a) to head (0a1c262).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #69   +/-   ##
========================================
  Coverage    75.00%   75.00%           
========================================
  Files            1        1           
  Lines            4        4           
========================================
  Hits             3        3           
  Misses           1        1           

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