use LiteralString on PydanticCustomError

[samuelcolvin]

to avoid temptation to make error_type and particularly message_template dynamic, might require a few type-ignores, but that's fine.

Noticed while reviewing pydantic/pydantic#5599.

[codspeed-hq]

CodSpeed Performance Report

Merging #562 PydanticCustomError-LiteralString (bd257d8) will not alter performances.

Summary

🔥 0 improvements
❌ 0 regressions
✅ 111 untouched benchmarks

🆕 0 new benchmarks
⁉️ 0 dropped benchmarks

[codecov-commenter]

Codecov Report

Merging #562 (bd257d8) into main (4f1ce33) will increase coverage by 0.07%.
The diff coverage is n/a.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
+ Coverage   94.13%   94.20%   +0.07%     
==========================================
  Files          94       94              
  Lines       12762    12762              
  Branches       24       24              
==========================================
+ Hits        12013    12022       +9     
+ Misses        743      734       -9     
  Partials        6        6              

see 1 file with indirect coverage changes

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