refactor: set Pattern encoder in ENCODERS_BY_TYPE

[PrettyWood]

Change Summary

Set Pattern encoder directly in ENCODERS_BY_TYPE to be used by fastapi

Related issue number

#2045

Well fastapi uses its own json encoder and reuses ENCODERS_BY_TYPE from pydantic here
https://github.com/tiangolo/fastapi/blob/5614b94ccc9f72f1de2f63aae63f5fe90b86c8b5/fastapi/encoders.py#L126-L127
We could maybe add Pattern directly in ENCODERS_BY_TYPE to make it work

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)

[samuelcolvin]

@PrettyWood I switched from a label to a milestone for v1.8.1, I think that makes more sense.

I'm not sure why this has been tagged as v1.8.1?

[PrettyWood]

@samuelcolvin It's not required for 1.8.1 but without it, the issue is not completely solved as it doesn't work for fastapi. And I thought making it work for fastapi was probably the most common usecase. See #2045 (comment)

[samuelcolvin]

this is great, thank you.