Update type annotation for create_model function

Updated create_model return type annotation to return type which inherits from __base__ argument.

For instance, before this PR mypy will fail on the code below:

from pydantic import BaseModel, create_model

class Point(BaseModel):
    x: int
    y: int

NewPoint = create_model('NewPoint', __base__=Point)

point = NewPoint.parse_obj(...)
point.x = 10  # mypy will fail here

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)