Remove pinning of Sphinx<4

[mgautierfr]

Fix #735

It seems the issue is a dependencies version problem. (See readthedocs/readthedocs.org#9038)
This issue is fixed with Sphinx 4.0.2 but we were forcing sphinx to be <4 (probably for the same kind of problem)
We could force Jinja2<3.0 as proposed in readthedocs issue.
However, I've decided to remove the Sphinx<4 to compile doc with up to date dependencies. But I'm open to counter-argument to force Jinja2<3.0

[codecov]

Codecov Report

Merging #736 (59364a7) into master (49f24d1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #736   +/-   ##
=======================================
  Coverage   58.31%   58.31%           
=======================================
  Files          56       56           
  Lines        3697     3697           
  Branches     2069     2069           
=======================================
  Hits         2156     2156           
  Misses       1540     1540           
  Partials        1        1           

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