fix(schema): JSON schema generation with a Literal of an enum member

[PrettyWood]

Change Summary

Support JSON schema generation with a Literal of an enum member

Related issue number

closes #2534
closes #2535

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 #2536 (accca08) into master (7b7e705) will not change coverage.
The diff coverage is 100.00%.

❗ Current head accca08 differs from pull request most recent head 0098070. Consider uploading reports for the commit 0098070 to get more accurate results

@@            Coverage Diff            @@
##            master     #2536   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5109      5110    +1     
  Branches      1050      1050           
=========================================
+ Hits          5109      5110    +1     

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

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[Kilo59]

Made some very minor comments, but LGTM.