Replace usage of deprecated datetime.utcnow() with datetime.now(UTC) (#360)

* Replace usage of deprecated datetime.utcnow() with datetime.now(UTC)

* fix: reorder imports for consistency in jwt.py and test_jwt.py

* fix: correct usage of datetime.now(UTC) in JWT validation functions

* fix: update datetime usage for compatibility with Python versions

Refactor datetime imports and usage to ensure compatibility with Python 3.12 and 3.13. Replace deprecated datetime.utcnow() with datetime.now(UTC) and handle ImportError for older versions.

* fix: update datetime usage for compatibility with Python 3.12 and 3.13

- Replace deprecated datetime.utcnow() with datetime.now(UTC) for better compatibility.
- Adjusted import statements and usage in jwt.py and test_jwt.py for consistency.