Get werkzeug version using importlib

Resolves deprecation warning:
```
.venv/lib/python3.11/site-packages/authlib/integrations/flask_oauth2/errors.py:4: DeprecationWarning: The '__version__' attribute is deprecated and will be removed in Werkzeug 3.1. Use feature detection or 'importlib.metadata.version("werkzeug")' instead.
    _version = werkzeug.__version__.split('.')[0]
```

authlib/integrations/flask_oauth2/errors.py

@@ -1,7 +1,7 @@
 import werkzeug
 from werkzeug.exceptions import HTTPException
 
-_version = werkzeug.__version__.split('.')[0]
+_version = importlib.metadata.version('werkzeug').split('.')[0]
 
 if _version in ('0', '1'):
     class _HTTPException(HTTPException):