update sphinx to 3.0.1

[bryceml]

What does this PR do?

tests running sphinx and lint on python3.7.6 with sphinx updated to 3.0.1

[alan-cugler]

https://github.com/saltstack/salt/blob/master/doc/conf.py#L12 needs to be changed to such

from sphinx.directives.other import TocTree

and then we can delete the duplicate concurrent, seeing as that is built into salt python 3
https://github.com/saltstack/salt/blob/master/doc/conf.py#L89
https://github.com/saltstack/salt/blob/master/doc/conf.py#L153

[bryceml]

These are the warnings I get when running locally without -W as a sphinx option:

building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1737 source files that are out of date
updating environment: [new config] 1737 added, 0 changed, 0 removed
reading sources... [100%] topics/yaml/index                                                                                                                                                                                                                                      
/code/doc/ref/configuration/minion.rst:713: WARNING: duplicate conf_master description of color_theme, other instance in ref/configuration/master
/code/doc/ref/configuration/minion.rst:2591: WARNING: duplicate conf_minion description of pillarenv_from_saltenv, other instance in ref/configuration/master
/code/salt/cloud/clouds/openstack.py:docstring of salt.cloud.clouds.openstack:1: WARNING: duplicate object description of salt.cloud.clouds.openstack, other instance in ref/clouds/all/salt.cloud.clouds.openstack, use :noindex: for one of them
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] topics/yaml/index                                                                                                                                                                                                                                       
/code/doc/glossary.rst:72: WARNING: term not in glossary: job
/code/doc/glossary.rst:195: WARNING: term not in glossary: highdata
/code/doc/ref/configuration/index.rst:7: WARNING: term not in glossary: master
/code/doc/ref/configuration/index.rst:7: WARNING: term not in glossary: minion
/code/doc/topics/beacons/index.rst:241: WARNING: term not in glossary: idempotent
/code/doc/topics/grains/index.rst:105: WARNING: term not in glossary: top file
/code/doc/topics/targeting/compound.rst:7: WARNING: term not in glossary: top file
/code/doc/topics/targeting/compound.rst:37: WARNING: term not in glossary: top file
/code/doc/topics/targeting/globbing.rst:14: WARNING: term not in glossary: minion id
/code/doc/topics/targeting/globbing.rst:21: WARNING: term not in glossary: minion id
/code/doc/topics/targeting/globbing.rst:21: WARNING: term not in glossary: top file
/code/doc/topics/targeting/globbing.rst:86: WARNING: term not in glossary: top file
/code/doc/topics/targeting/index.rst:15: WARNING: term not in glossary: top file
/code/doc/topics/targeting/nodegroups.rst:65: WARNING: term not in glossary: top file
/code/doc/topics/targeting/nodegroups.rst:70: WARNING: term not in glossary: top file
/code/doc/topics/tutorials/http.rst:186: WARNING: Could not lex literal_block as "python". Highlighting skipped.
/code/doc/topics/tutorials/modules.rst:12: WARNING: term not in glossary: master
/code/doc/topics/tutorials/modules.rst:12: WARNING: term not in glossary: minion
/code/doc/topics/tutorials/pillar.rst:142: WARNING: term not in glossary: top file
generating indices...  genindex py-modindexdone
writing additional pages...  404 searchdone
copying images... [100%] _static/spm-package-extraction.png                                                                                                                                                                                                                      
copying static files... ... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 22 warnings.

[bryceml]

I think I fixed all the warnings, we'll see.