[doc] Update the required Sphinx version in conf.py

[teojgo]

Fixes the build failure on readthedocs

[vkarak]

Aha, and I was always wondering why rtd was not using a more recent version of Sphinx :-)

[codecov-commenter]

Codecov Report

Merging #1327 into master will decrease coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
- Coverage   91.73%   91.44%   -0.30%     
==========================================
  Files          83       83              
  Lines       12486    12058     -428     
==========================================
- Hits        11454    11026     -428     
  Misses       1032     1032              

Impacted Files	Coverage Δ
...tests/resources/checks_unlisted/deprecated_test.py	53.84% <0.00%> (-3.30%)	⬇️
reframe/core/systems.py	86.06% <0.00%> (-2.13%)	⬇️
reframe/core/modules.py	60.51% <0.00%> (-1.93%)	⬇️
reframe/core/deferrable.py	92.94% <0.00%> (-1.79%)	⬇️
unittests/test_launchers.py	92.72% <0.00%> (-1.48%)	⬇️
unittests/test_environments.py	72.07% <0.00%> (-1.22%)	⬇️
reframe/core/exceptions.py	85.07% <0.00%> (-0.75%)	⬇️
reframe/core/schedulers/slurm.py	55.04% <0.00%> (-0.68%)	⬇️
reframe/core/environments.py	95.00% <0.00%> (-0.66%)	⬇️
unittests/test_containers.py	96.72% <0.00%> (-0.56%)	⬇️
... and 33 more

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[vkarak]

lgtm