Test django-splash on Python 3.11

[feanil]

This repository is a depedency of edx-platform and needs to be upgraded to Python 3.11 before
the Readwood release is cut (mid-April).

- [ ] Requirements are compiled with Python 3.8
- [ ] Tests are run on Python 3.8 and 3.11
- [ ] (Optional) Tests are also run with 3.12 and passing or 3.12 issues are ticketed.
- [ ] Classifiers in setup.py setup.cfg or pyproject.toml indicate Python 3.11 support
- [ ] A new version is release to PyPI
- [ ] A PR is merged to edx-platform to use the new version

• Finish the django-splash DEPR instead.

[ormsbee]

I wrote this in the django-splash PR, but django-splash should be archived and removed from edx-platform usage/dependencies instead of upgraded.

[feanil]

Instead of upgrading this, we'll be removing it via openedx/edx-platform#34528