Add Config.keep_untouched for custom descriptors support

[Bobronium]

Change Summary

This will allow to extend default TYPE_BLACKLIST during class creation.

It's very helpful if we want use custom descriptors for our class (like cached_property, etc) and don't want pydantic to break them.

from pydantic import BaseModel

class _ClassPropertyDescriptor:
    __slots__ = ('getter', )

    def __init__(self, getter):
        self.getter = getter

    def __get__(self, instance, owner):
        return self.getter(owner)

classproperty = _ClassPropertyDescriptor

class Model(BaseModel):
    class Config:
        keep_untouched = (classproperty, )

    @classproperty
    def class_name(cls) -> str:
        return cls.__name__

print(Model.class_name)  # Model
print(Model().class_name)  # Model

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

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The diff coverage is 100%.

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[samuelcolvin]

otherwise LGTM.

[samuelcolvin]

please rename TYPE_BLACKLIST to UNTOUCHED_TYPES to be consistent with config.

[Bobronium]

Done :)