Run RTD build using pinned Sphinx version

[amotl]

Out of curiosity, this checks the RTD build using a pinned Sphinx version. This is all about investigating the default/non-default behavior related to readthedocs/readthedocs.org#6296.

The background on that is that the RTD build says it is Running Sphinx v1.8.5 [1] when running on a loose constraint like Sphinx>=1.8.5,<4 as defined within crate-docs-theme [2], which is pulled in as a dependency.

[1] https://readthedocs.org/projects/crate-docs/builds/12559570/
[2] https://github.com/crate/crate-docs-theme/blob/0.12.0/setup.py#L56

[amotl]

Indeed, only when explicitly pinning to sphinx==3.3.1 here, the RTD build says it is Running Sphinx v3.3.1 [3]. However, @mechanomi also just suggested that "wiping" the RTD environment would also help, so nevermind.

[3] https://readthedocs.org/projects/crate-docs/builds/12559602/

[amotl]

Still, even after wiping the RTD latest environment at [4] and after pushing to #50, it says Running Sphinx v1.8.5 [5]. Let's come back to this later.

[4] https://readthedocs.org/projects/crate-docs/versions/
[5] https://readthedocs.org/projects/crate-docs/builds/12559720/

[amotl]

Now, I've pinned the demo docs project to Sphinx 3.5.2, and, indeed, [1] says it is Running Sphinx v3.5.2. By doing it that way, we are following the recommendations of the Reproducible Builds documentation of Read the Docs, specifically the Pinning dependencies section [2].

[1] https://readthedocs.org/api/v2/build/13282535.txt
[2] https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#pinning-dependencies

[amotl]

With fc29c7f, we are also using an explicitly pinned Sphinx version within the common-build requirements.txt.