Closes #19248: actually check for Python 3.x in tools/sphinx-build.py.

Doc/tools/sphinx-build.py

@@ -15,7 +15,7 @@ warnings.filterwarnings('ignore', category=UserWarning, module='jinja2')
 
 if __name__ == '__main__':
 
-    if sys.version_info[:3] < (2, 4, 0):
+    if sys.version_info[:3] < (2, 4, 0) or sys.version_info[:3] > (3, 0, 0):
         sys.stderr.write("""\
 Error: Sphinx needs to be executed with Python 2.4 or newer (not 3.0 though).
 (If you run this from the Makefile, you can set the PYTHON variable