Resolve deprecation warning related to using datetime.utcnow in py3.12

Starting from python 3.12.0, we started seeing some deprecation warning related to use of `datetime.datetime.utcnow`:

```
/Users/.../lib/python3.12/site-packages/fastapi_jwt/jwt.py:135: 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).
now = datetime.utcnow()
```

This PR should provide the same level of functionality without firing a deprecation warning, by implementing the suggested change.

fastapi_jwt/jwt.py

@@ -1,5 +1,5 @@
 from abc import ABC
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, UTC
 from typing import Any, Dict, Optional, Set
 from uuid import uuid4
 
@@ -132,7 +132,7 @@ def _generate_payload(
         unique_identifier: str,
         token_type: str,
     ) -> Dict[str, Any]:
-        now = datetime.utcnow()
+        now = datetime.now(UTC)
 
         return {
             "subject": subject.copy(),  # main subject