Fix mypy plugin issue with self field

[uriyyo]

Fix mypy plugin issue with self field.

Currently mypy plugin will fail at this code:

from pydantic import BaseModel


class Model(BaseModel):
    self: str

error: Name 'self' already defined (possibly by an import)  [no-redef]

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

[uriyyo]

please review

[PrettyWood]

Thanks! 🙏