docs: sphinx >=4.0 breaks build

[dmsimard]

What is the issue ?

Sphinx recently released v4 and it broke ara's doc builds, for example with tox -e docs:

Running Sphinx v4.0.2
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 15 source files that are out of date
updating environment: [new config] 15 added, 0 changed, 0 removed
reading sources... [  6%] ansible-configuration
reading sources... [ 13%] ansible-plugins-and-use-cases
reading sources... [ 20%] api-configuration
reading sources... [ 26%] api-documentation
reading sources... [ 33%] api-security
reading sources... [ 40%] api-usage
reading sources... [ 46%] changelog-release-notes
reading sources... [ 53%] cli
reading sources... [ 60%] container-images
reading sources... [ 66%] contributing
reading sources... [ 73%] distributed-sqlite-backend
reading sources... [ 80%] faq
reading sources... [ 86%] getting-started
reading sources... [ 93%] index
reading sources... [100%] troubleshooting

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [  6%] ansible-configuration
writing output... [ 13%] ansible-plugins-and-use-cases

Exception occurred:
  File "/home/zuul/src/github.com/ansible-community/ara/.tox/docs/lib/python3.6/site-packages/sphinx/highlighting.py", line 127, in get_lexer
    lexer = lexer_classes[lang](**opts)
TypeError: 'AnsibleYamlLexer' object is not callable

What should be happening ?

I haven't yet investigated the issue but it should work

[dmsimard]

Will be fixed once #290 merges.