Fix sphinx-build so that it runs with 2.x and 3.x but refuses to start with 3.x.

Doc/tools/sphinx-build.py

@@ -11,13 +11,12 @@ import sys
 
 if __name__ == '__main__':
 
-    if sys.version_info[:3] < (2, 5, 1):
-        print("""\
-Error: Sphinx needs to be executed with Python 2.5.1 or newer
-(If you run this from the Makefile, you can set the PYTHON variable
-to the path of an alternative interpreter executable, e.g.,
-``make html PYTHON=python2.5``).
-""", file=sys.stderr)
+    if not (2, 5, 1) <= sys.version_info[:3] < (3, 0, 0):
+        sys.stderr.write("""\
+Error: Sphinx needs to be executed with Python 2.5.1 or newer (not 3.0 though).
+If you run this from the Makefile, you can set the PYTHON variable to the path
+of an alternative interpreter executable, e.g., ``make html PYTHON=python2.5``.)
+""")
         sys.exit(1)
 
     from sphinx import main