Change Summary

I find it a bit unusual to have the following working:

from pydantic import BaseModel, Field

class Model(BaseModel):
    a: int = 1
    model_config = {"frozen": True}

m = Model()
m.a = 2
#> Raises ValidationError

But not the following:

from pydantic import BaseModel, Field

class Model(BaseModel):
    a: int = Field(frozen=True)

m = Model(a=1)
m.a = 2
#> No ValidationError, unless model_config.validate_assignment is set to True

If you think the current behavior is as designed, I can revert my commit but I think a note on the Field.frozen docstring would be required.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin