Change testing to only cover latest sphinx version

[foster999]

Remove sphinx version pinning - only the latest version of sphinx will be fully supported, but previous versions will work if sphinx dependencies (i.e. jinja2) are managed correctly. This is inline with the approach at sphinx
Remove tests that were specific to older versions of sphinx and pygments
Remove jinja2 version pinning, as this is pinned in latest version of sphinx
Added weekly scheduled testing to catch breaking changes in unpinned dependencies
Updated docutils pin to allow use of verison 0.18.x

Will resolve #166 and will resolve #169

[codecov]

Codecov Report

Merging #167 (52bbb21) into master (b9fd7d8) will increase coverage by 4.57%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   92.69%   97.27%   +4.57%     
==========================================
  Files           2        2              
  Lines         219      220       +1     
==========================================
+ Hits          203      214      +11     
+ Misses         16        6      -10     

Flag	Coverage Δ
pytests	97.27% <ø> (+4.57%)	⬆️

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Impacted Files	Coverage Δ
sphinx_tabs/tabs.py	97.24% <0.00%> (+4.62%)	⬆️

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[foster999]

Following the changes yesterday, I'm now planning on only testing against the latest sphinx and pygments versions here. This will simplify the testing outcomes and help us to be a bit more reactive with new versions of our dependencies. @Daltz333 does this seem reasonable?