fix(event-handler): add missing from __future__ import annotations in oauth2.py

Add `from __future__ import annotations` to enable PEP 604 union syntax (X | None) for Python versions below 3.10.

Fixes FA102 ruff lint errors:
- aws_lambda_powertools/event_handler/openapi/swagger_ui/oauth2.py:50

Part of #8088

Signed-off-by: hirenkumar-n-dholariya <hirenkumarnd@gmail.com>

Author: hirenkumar-n-dholariya

aws_lambda_powertools/event_handler/openapi/swagger_ui/oauth2.py

@@ -1,4 +1,6 @@
 # ruff: noqa: E501 FA100
+from __future__ import annotations
+
 import warnings
 from typing import Sequence