Change readthedocs python version

[giffels]

readthedocs is using Python3.7 to build the documentation by default, which now failing since we require a minium python version of 3.8. This pull request fixes the failing readthedocs build by requesting a build using Python 3.11.

In addition, the build seem to fail with sphinx==7, so that we now require sphinx<7 in setup.py. See readthedocs/readthedocs.org#10279 for details.

In addition, the rendering of the two column layout seems to have problems with newer version of docutils. Therefore, we will fix docutils<0.17 for the time being. Thanks to @mschnepf for the report.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (aa30b0c) 98.90% compared to head (daecc93) 98.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #310   +/-   ##
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  Coverage   98.90%   98.90%           
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  Files          55       55           
  Lines        2277     2277           
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  Hits         2252     2252           
  Misses         25       25           

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[giffels]

@rfvc @RHofsaess could you review again, please? By fixing docutils<0.17 also sphinx==5.3.0 is installed, so that I removed that constraint.