setup: make script executable with python3

Replace the incompatible print statement. Writing a warning to stderr is a good
idea anyway.

setup.py

@@ -44,7 +44,7 b' except:'
     pass
 
 if isironpython:
-    print "warning: IronPython detected (no bz2 support)"
+    sys.stderr.write("warning: IronPython detected (no bz2 support)\n")
 else:
     try:
         import bz2