Fix support for using a subclass of an annotation as a default

[JacobHayes]

Change Summary

A field can be set with a default that is a subclass of the type hint when set together:

class MyStr(str):
    pass

class Simple(BaseModel):
    x: str = MyStr("test")

Additionally, a model can be instantiated with a subclass of the type hint: Simple(x=MyStr("works")).

However, when using model subclasses where the base sets a type hint, but the default (eg: MyStr) is set in a
subclass, the The type of {name}.{var_name} differs from the new default value exception is raised:

class MyStr(str):
    pass

class Base(BaseModel):
    x: str

class Sub(Base):
    x = MyStr("test")

I think rather than checking for an exact match on the type, we should be able to use lenient_issubclass. This seems pretty sane and didn't break any of the existing tests. My use case is actually using an Annotation(BaseModel) as the type hint, but setting subclass defaults like Vendor(Annotation), but that should follow from this simpler test case.

Related issue number

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)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[JacobHayes]

Please review when you have a moment. The change is a bit simpler ignoring whitespace changes.

[PrettyWood]

Makes sense! Thank you 🙏