fix: do not overwrite declared __hash__ in subclasses of a model

[PrettyWood]

Change Summary

Declared __hash__ was overwritten and not passed to child models

Related issue number

closes #2422

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2423 (7431944) into master (a8d50ae) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2423   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5080      5084    +4     
  Branches      1041      1042    +1     
=========================================
+ Hits          5080      5084    +4     

Impacted Files	Coverage Δ
pydantic/main.py	100.00% <100.00%> (ø)

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[samuelcolvin]

great, thank you. What else needs to be included in v1.8.1?