removed readthedocs_ext from requirements

[marscher]

The Sphinx extension "readthedocs_ext" is kind of deprecated (archived repo) and not compatible with the latest release of Sphinx.
Therefor we remove it from our doc building process.

Fixes #1339

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.48%. Comparing base (1ae3814) to head (24ac30b).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   86.59%   86.48%   -0.12%     
==========================================
  Files         113      113              
  Lines       10496    10513      +17     
  Branches     4029     4029              
==========================================
+ Hits         9089     9092       +3     
- Misses        807      822      +15     
+ Partials      600      599       -1     

see 2 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.