Add create_model overload

Currently mypy will fail on this code:

from pydantic import create_model

Foo = create_model("Foo")

error: Need type annotation for 'Foo'  [var-annotated]

With this PR mypy will correctly resolve return value type of create_model:

from pydantic import create_model

Foo = create_model("Foo")
reveal_type(Foo)

note: Revealed type is 'Type[pydantic.main.BaseModel]'

And it will still work for a cases when __base__ is specified:

from pydantic import create_model, BaseModel


class Bar(BaseModel):
    a: int


Foo = create_model("Foo", __base__=Bar)
reveal_type(Foo)

note: Revealed type is 'Type[temp.Bar*]'

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