configure: Add checking python3-sphinx

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
  sphinx-build` executable
* 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.