Move const validator to post validators

[selimb]

Change Summary

Move const validator to post validators so that it validates the parsed value, not the input value.

Related issue number

Fixes #1410

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/changes/1410-selimb.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1446 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1446   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3747      3747           
  Branches       742       742           
=========================================
  Hits          3747      3747           

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

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[selimb]

Let me know if you want something added to the docs. Wasn't sure how best to phrase it to stay consistent with the existing style.

[samuelcolvin]

thanks so much, I think this is what people would expect so no change to the docs required.

[selimb]

Yeah that's sorta what I figured. Welcome! Thanks again for this project. Cheers.