Pin sphinx to <5.1.0

[eivindjahren]

sphinx-5.1.0 broke our build. Issue raised here:
sphinx-doc/sphinx#10701. Pinning for now.

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #3627 (22ffa34) into main (ec015e8) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3627      +/-   ##
==========================================
- Coverage   64.76%   64.74%   -0.03%     
==========================================
  Files         601      601              
  Lines       46529    46529              
  Branches     4186     4186              
==========================================
- Hits        30136    30124      -12     
+ Misses      15041    14953      -88     
- Partials     1352     1452     +100     

Impacted Files	Coverage Δ
ert_gui/ertwidgets/validationsupport.py	79.45% <0.00%> (-19.18%)	⬇️
ert_gui/ertwidgets/__init__.py	75.00% <0.00%> (-2.28%)	⬇️
ert/data/record/_transformation.py	88.57% <0.00%> (-0.48%)	⬇️
libres/lib/enkf/field.cpp	0.70% <0.00%> (ø)
libres/lib/config/conf.cpp	60.47% <0.00%> (ø)
libres/lib/enkf/gen_kw.cpp	44.55% <0.00%> (ø)
libres/lib/python/init.cpp	22.58% <0.00%> (ø)
libres/lib/enkf/surface.cpp	2.59% <0.00%> (ø)
libres/lib/rms/rms_file.cpp	0.00% <0.00%> (ø)
libres/lib/enkf/enkf_obs.cpp	72.49% <0.00%> (ø)
... and 41 more

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more