fix: type hints on BaseConfig

[PrettyWood]

Change Summary

Just added SchemaExtraCallable when TYPE_CHECKING is False to make sure typing.get_type_hints does not crash.

Related issue number

closes #1614

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 #1618 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1618   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3793      3796    +3     
  Branches       752       752           
=========================================
+ Hits          3793      3796    +3     

Impacted Files	Coverage Δ
pydantic/schema.py	100.00% <0.00%> (ø)
pydantic/validators.py	100.00% <0.00%> (ø)

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[samuelcolvin]

great, thank you.

[pawamoy]

Thank you @PrettyWood!