fix: avoid RecursionError when using some types like Enum or Literal with generic models

[PrettyWood]

Change Summary

Since an Enum is an Iterable, which would cause an infinite loop.

Related issue number

closes #2436
closes #2454

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 #2438 (f5c24cb) into master (a8d50ae) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2438   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5080      5100   +20     
  Branches      1041      1048    +7     
=========================================
+ Hits          5080      5100   +20     

Impacted Files	Coverage Δ
pydantic/generics.py	100.00% <100.00%> (ø)
pydantic/json.py	100.00% <0.00%> (ø)
pydantic/main.py	100.00% <0.00%> (ø)
pydantic/types.py	100.00% <0.00%> (ø)
pydantic/fields.py	100.00% <0.00%> (ø)
pydantic/_hypothesis_plugin.py	100.00% <0.00%> (ø)

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[samuelcolvin]

LGTM, @PrettyWood & @daviskirk I'll merge this soon if you're happy with it.

[PrettyWood]

Thanks @samuelcolvin ! 🙏

I just updated the change description since it also solves #2454

[samuelcolvin]

We should add a test for a literal too, just so we don't forget.

[daviskirk]

Looks great to me thanks for fixing my oversights, just tried it out with the literal, works fine as well!

@skip_36
def test_generic_literal():
    T = TypeVar('T')

    class SomeGenericModel(GenericModel, Generic[T]):
        some_field: T

    class MyModel(BaseModel):
        my_gen: SomeGenericModel[Literal["foo"]]

    m = MyModel.parse_obj({'my_gen': {'some_field': "foo"}})
    assert m.my_gen.some_field == "foo"

    with pytest.raises(ValidationError, match=".*permitted: 'foo'.*"):
        MyModel.parse_obj({'my_gen': {'some_field': "bar"}})

edit: @PrettyWood, you're really quick! Please Ignore my comment

[samuelcolvin]

🙏 thanks everyone.