Update doc footer/copyright

[timhoffm]

• Target the master branch for bug fixes, documentation updates and trivial changes.
  Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

This PR includes a few small doc updates:

• remove an unused code line and extra space introduced by change doc copyright year to range and auto update year #3476 / Do not claim copyright for future years #3710.
• update the year in the license
• update the link to sphinx in the footer
• add the sphinx version in the footer (This is taken from default sphinx; I find it really helpful to know which sphinx version is used when trying to improve the docs).

[nicoddemus]

Thanks!

[codecov]

Codecov Report

Merging #5569 into master will increase coverage by 2.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5569      +/-   ##
==========================================
+ Coverage   93.83%   96.09%   +2.25%     
==========================================
  Files         117      117              
  Lines       25713    25713              
  Branches     2493     2493              
==========================================
+ Hits        24128    24708     +580     
+ Misses       1256      701     -555     
+ Partials      329      304      -25

Impacted Files	Coverage Δ
src/_pytest/fixtures.py	97.52% <0%> (+0.13%)	⬆️
src/_pytest/python.py	93% <0%> (+0.24%)	⬆️
testing/test_config.py	99.83% <0%> (+0.33%)	⬆️
src/_pytest/config/__init__.py	94.62% <0%> (+0.48%)	⬆️
src/_pytest/assertion/rewrite.py	96.08% <0%> (+0.53%)	⬆️
testing/test_pytester.py	89.27% <0%> (+0.57%)	⬆️
testing/test_runner.py	96.27% <0%> (+0.65%)	⬆️
src/_pytest/doctest.py	94.38% <0%> (+0.74%)	⬆️
src/_pytest/nodes.py	95.5% <0%> (+1%)	⬆️
testing/test_terminal.py	99.84% <0%> (+1.4%)	⬆️
... and 19 more

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