Test Python 3.9

BinderHub is broken on Python 3.8:

```
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/runner/work/binderhub/binderhub/binderhub/__main__.py", line 2, in <module>
    from binderhub.app import main
  File "/home/runner/work/binderhub/binderhub/binderhub/app.py", line 45, in <module>
    from .base import VersionHandler
  File "/home/runner/work/binderhub/binderhub/binderhub/base.py", line 13, in <module>
    from .utils import ip_in_networks
  File "/home/runner/work/binderhub/binderhub/binderhub/utils.py", line 172, in <module>
    ip_addr: str, networks: Iterable[ipaddress.IPv4Network | ipaddress.IPv6Network]
```

.github/workflows/test.yml

@@ -412,7 +412,7 @@ jobs:
         id: setup-python
         with:
           # Set this to the oldest Python version supported by BinderHub
-          python-version: "3.8"
+          python-version: "3.9"
 
       - name: Cache pip
         uses: actions/cache@v4