Revert "Pin sphinx to <5.1.0"

[eivindjahren]

Issue with sphinx that caused the pin was resolved in 5.1.1

This reverts commit 6e48689.

Upstream issue: sphinx-doc/sphinx#10701

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 #3636 (e1f058c) into main (1d5313d) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3636      +/-   ##
==========================================
+ Coverage   64.74%   64.77%   +0.02%     
==========================================
  Files         601      601              
  Lines       46528    46528              
  Branches     4186     4186              
==========================================
+ Hits        30125    30137      +12     
+ Misses      15051    14941     -110     
- Partials     1352     1450      +98     

Impacted Files	Coverage Δ
res/job_queue/forward_model_status.py	88.60% <0.00%> (-5.07%)	⬇️
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)
libres/lib/enkf/gen_data.cpp	6.39% <0.00%> (ø)
libres/lib/enkf/time_map.cpp	48.32% <0.00%> (ø)
... and 39 more

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