Fix builds on readthedocs.

[eirrgang]

Improve dependency management for docs builds.

• Fix the source of the napoleon sphinx extension.
• Use pip-tools to generate docs/requirements.txt
• Update the configuration for readthedocs.org

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2a02fb8) 72.64% compared to head (6fef8ad) 72.64%.

:exclamation: Current head 6fef8ad differs from pull request most recent head 7906d08. Consider uploading reports for the commit 7906d08 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #342 +/- ## ======================================= Coverage 72.64% 72.64% ======================================= Files 28 28 Lines 3520 3520 Branches 441 441 ======================================= Hits 2557 2557 Misses 757 757 Partials 206 206 ```

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