MutableMapping error message

```
excalibur initdb
Traceback (most recent call last):
File "/home/boss/.local/bin/excalibur", line 5, in
from excalibur.cli import cli
File "/home/boss/.local/lib/python3.11/site-packages/excalibur/cli.py", line 7, in
from . import __version__, settings
File "/home/boss/.local/lib/python3.11/site-packages/excalibur/settings.py", line 10, in
from . import configuration as conf
File "/home/boss/.local/lib/python3.11/site-packages/excalibur/configuration.py", line 7, in
from backports.configparser import ConfigParser
File "/home/boss/.local/lib/python3.11/site-packages/backports/configparser/__init__.py", line 130, in
from collections import MutableMapping
ImportError: cannot import name 'MutableMapping' from 'collections' (/usr/lib/python3.11/collections/__init__.py)
```

Start by reproducing `excalibur initdb` with Python 3.11 and read `excalibur/configuration.py`, then inspect the reported `backports.configparser` import in the traceback. Done means the command gets past startup without the `MutableMapping` ImportError and the relevant compatibility behavior is covered by the project’s existing checks.

Written by the indexing model from the issue text.