Apply __modify_schema__ on enum schema rather than fields that use it

[therefromhere]

Change Summary

Make __modify_schema__ on enums apply to the enum schema rather than fields that use the enum.

Resolves #1576

Does this seem like a reasonable thing to do?

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 #1581 into master will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##            master    #1581      +/-   ##
===========================================
- Coverage   100.00%   99.94%   -0.06%     
===========================================
  Files           21       21              
  Lines         3793     3802       +9     
  Branches       752      757       +5     
===========================================
+ Hits          3793     3800       +7     
- Misses           0        1       +1     
- Partials         0        1       +1     

Impacted Files	Coverage Δ
pydantic/schema.py	100.00% <100.00%> (ø)
pydantic/networks.py	99.21% <0.00%> (-0.79%)	⬇️
pydantic/types.py	100.00% <0.00%> (ø)
pydantic/fields.py	100.00% <0.00%> (ø)
pydantic/typing.py	100.00% <0.00%> (ø)
pydantic/validators.py	100.00% <0.00%> (ø)
pydantic/dataclasses.py	100.00% <0.00%> (ø)
pydantic/env_settings.py	100.00% <0.00%> (ø)
pydantic/class_validators.py	100.00% <0.00%> (ø)

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