Convert datetime.utcnow -> datetime.now(UTC)

[psi29a]

Removes deprecation warning and makes a timezone (UTC) aware datetime object.

  /Users/psi29a/Workspace/backend/venv/lib/python3.12/site-packages/pybreaker/__init__.py:257: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    self._state_storage.opened_at = datetime.utcnow()

  /Users/psi29a/Workspace/backend/venv/lib/python3.12/site-packages/pybreaker/__init__.py:766: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    if opened_at and datetime.utcnow() < opened_at + timeout:
    ```

[danielfm]

@psi29a There is a lint error in this PR. You can fix that by putting the UTC first in the import list.

[psi29a]

Sure! :)