Temporarily require sphinx < 7.3.0

Description

Temporarily require sphinx < 7.3.0

Sphinx-immaterial is not yet compatible with sphinx >= 7.3.0. See https://github.com/jbms/sphinx-immaterial/issues/345.

Checklist

Thanks for your contribution! Please complete and check off the following quality control tasks.

Changes:

• I created this merge request from my own fork and from a branch other than main or master.
• I checked that the CI pipeline completes without errors.
• I added a change log entry (or this is a minor change that does not require a change log entry).

Whitespace:

• No line that I added ends with trailing whitespace.
• Every file that I modified ends with one blank line.

Grammar and spelling:

• I proofread my contribution and made sure that it is free from spelling and grammatical errors.
• I used American English rather than British English spelling.
• I used the Oxford (serial) comma in lists of three or more items.
• I minimized use of the passive voice.

Figures and code:

• I contributed plot directive code to generate figures (or I did not contribute any figures).
• I contributed Python code that conforms to the PEP 8 style guide (or I did not contribute Python code).