fix ValidationError.json()

[samuelcolvin]

Change Summary

Fix JSON serialization errors on ValidationError.json() by using pydantic_encoder.

Checklist

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

[codecov]

Codecov Report

Merging #922 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #922   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          16     16           
  Lines        2799   2800    +1     
  Branches      543    543           
=====================================
+ Hits         2799   2800    +1

Impacted Files	Coverage Δ
pydantic/error_wrappers.py	100% <100%> (ø)	⬆️

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