Add the missing install requirement packaging (#225)

The `packaging` lib is not listed in the `install_requires` list.

https://github.com/greyli/flask-extension-status/actions/runs/7129071225/job/19412328108

```
Run python -c "from flask import Flask; app = Flask(__name__); from flask_debugtoolbar import DebugToolbarExtension; DebugToolbarExtension(app)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/flask_debugtoolbar/__init__.py", line 6, in <module>
    from packaging import version as version_builder
ModuleNotFoundError: No module named 'packaging'
Error: Process completed with exit code 1.
```

setup.py

@@ -9,5 +9,6 @@
         'itsdangerous',
         'werkzeug',
         'MarkupSafe',
+        'packaging',
     ],
 )