This checks the configuration about python3-sphinx.
We need python3-sphinx instead of python2-sphinx to build documentation.
The approach is as follows:
* Check python3 version with custom `conf.py` invoked from
* Place custom `conf.py` into new `utils/check-sphinx` directory
If sphinx is for python2 not python3, it's treated as config ERROR
instead of WARN.
See also #17346 and #17356.