configure: Add checking python3-sphinx

This is a continuation of !1989 (closed).

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 (closed) and #17356 (closed).