Fix error in documentation build

[tephyr]

Also update minimum Sphinx version to 4.

ReadTheDocs builds have been failing for a while (latest error log as of this PR, for 2022-05-09). Root cause appears to be an outdated reference to a Sphinx extension. Correct library is installed, but referenced with the wrong name.

Contributor Checklist:

• [N/A ] I have updated the unit tests
• [ Y] I have created a file in the newsfragments directory (and read the README.txt in that directory)
• [ Y] I have updated the appropriate documentation

[codecov]

Codecov Report

Merging #6505 (9a40e45) into master (42e9ec4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6505   +/-   ##
=======================================
  Coverage   92.06%   92.07%           
=======================================
  Files         343      343           
  Lines       38616    38616           
=======================================
+ Hits        35553    35554    +1     
+ Misses       3063     3062    -1     

Impacted Files	Coverage Δ
worker/buildbot_worker/tunnel.py	89.83% <0.00%> (+1.69%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 42e9ec4...9a40e45. Read the comment docs.

[p12tic]

Thanks a lot!