Pin Sphinx<9 for doc builds

[chrizzFTD]

• Pin sphinx>=7.2,<9 as 9 causes a failure:

   sphinx.errors.ExtensionError: Handler <function deprecated_configs_warning at 0x7c5433104fe0> for event 'config-inited' threw an exception (exception: cannot unpack non-iterable _Opt object)

  which comes through sphinx-hoverxref (which has been archived) relying on _Opt config API.

• Local docs build: pip install -e .[docs,create] then python docs/source/build_docs.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.73%. Comparing base (655b0f7) to head (7484980).

❗ There is a different number of reports uploaded between BASE (655b0f7) and HEAD (7484980). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (655b0f7)	HEAD (7484980)
	6	1

Additional details and impacted files

@@             Coverage Diff              @@
##           develop      #49       +/-   ##
============================================
- Coverage    89.37%   73.73%   -15.65%     
============================================
  Files           13       13               
  Lines         3088     3088               
============================================
- Hits          2760     2277      -483     
- Misses         328      811      +483     

see 8 files with indirect coverage changes

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