Utilities/Sphinx: Restore explicit check for Sphinx 2.x or later

Refactoring in commit adbc8c982d (Utilities/Sphinx: Fix flake8 gripes in
cmake.py, 2023-03-13, v3.27.0-rc1~317^2) moved the assertion added by
commit cef51925a4 (Utilities/Sphinx: Require Sphinx 2.x or later,
2023-03-13, v3.27.0-rc1~317^2~2) to after imports, which is too late to
clearly reject older Sphinx versions.