fix compat. w/ python < 3.3

wscript

@@ -167,7 +167,7 @@ def configure(conf):
     # this causes pybindgen/version.py to be generated by setuptools_scm
     subprocess.Popen(
         [conf.env["PYTHON"][0], "setup.py", "--help-commands"],
-        stdout=subprocess.DEVNULL,
+        stdout=getattr(subprocess, "DEVNULL", None),
     ).wait()
 
     try: