Error with Sphinx 5.1.1

Running `travis-sphinx build --source=doc` results in
```py
Traceback (most recent call last):
File "/home/jayvdb/python/sphinx-rust/.tox/py310/lib/python3.10/site-packages/sphinx/config.py", line 347, in eval_config_file
exec(code, namespace)
File "/home/jayvdb/python/sphinx-rust/doc/conf.py", line 4, in
import sphinxrust
File "/home/jayvdb/python/sphinx-rust/.tox/py310/lib/python3.10/site-packages/sphinxrust/__init__.py", line 3, in
from .rustdomain import RustDomain
File "/home/jayvdb/python/sphinx-rust/.tox/py310/lib/python3.10/site-packages/sphinxrust/rustdomain.py", line 22, in
from sphinx.locale import l_
ImportError: cannot import name 'l_' from 'sphinx.locale' (/home/jayvdb/python/sphinx-rust/.tox/py310/lib/python3.10/site-packages/sphinx/locale/__init__.py)
```

Reproduce the failure with `travis-sphinx build --source=doc`, then inspect `doc/conf.py` and `sphinxrust/rustdomain.py`, especially the import reported in the traceback. Done means the documentation build completes successfully with Sphinx 5.1.1.

Written by the indexing model from the issue text.