Upgrade to latest sphinx version

[n2ygk]

Is your feature request related to a problem? Please describe.

Builds started failing for pinned Sphinx<3 because of an unpinned requirement for Jinja2.
This highlights that we are well below the latest release of Sphinx 4.x so it's probably time to suck it up and figure out what's needed to upgrade.

Describe the solution you'd like

Get tox -e docs working with current versions of Sphinx, etc.

Describe alternatives you've considered

Continue to live with old pinned versions. (Jinja2 pinned version is added in #1134).

Additional context

[mokuhasushi]

Hi, I believe the issue to be related to the package m2r.
The proposed fix is to switch to m2r2 as the latest release of m2r is dated October 2018.

Other than m2r > m2r2 is it ok to remove the pin on Jinja2 here? (Jinja2<3.1 > Jinja2)

[andreasofthings]

Indeed it is, I just pushed a branch latest-version-of-sphinx. Let me check the output later today before opening a pull request.