Make signature class only

[Bobronium]

Change Summary

Make __signature__ attribute class-only

Related issue number

Closes #1419

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 #1466 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1466   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          21       21           
  Lines        3761     3770    +9     
  Branches      748      749    +1     
=======================================
+ Hits         3759     3768    +9     
  Misses          1        1           
  Partials        1        1           

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

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[samuelcolvin]

Very clever solution. LGTM 👍

@JrooTJunior @PrettyWood does this work for you?

[PrettyWood]

Yes I already said it was a way nicer and efficient solution on the PR I closed ! Thanks

[JrooTJunior]

Looks good

[basnijholt]

In case someone else finds this issue when investigating why the signature of

from pydantic import BaseModel

class FooModel(BaseModel):
    id: int
    name: str = None
    description: str = 'Foo'

is Init signature: FooModel(__pydantic_self__, **data: typing.Any) -> None.

You might be running into this bug napari/napari#2264, which is a strange interaction between PySide2 and Pydantic.

Just leaving this here because it took me a long time to find out what's happening.