Update docs dependencies

[sirosen]

Currently doc builds appear broken. I think this is producing a poor signal:noise on this repo and probably making it harder for maintainers to handle PRs.

pip-compile -U requirements/docs.in to get latest. Importantly, this picks up pallets-sphinx-themes==2.0.2 so that doc builds now work with the latest jinja2 versions.

[sirosen]

I just realized that #1019 has been sitting, so possibly this should be closed in favor of several of the dependabot PRs. I'll leave this here though, in case this is more convenient.

[davidism]

Just did this manually for everything.