Stop using PyMem_FREE while the GIL is not held.  For details see:
http://mail.python.org/pipermail/python-dev/2007-May/072896.html
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 29ef1dc..a63a77a 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6296,8 +6296,8 @@
 #else
 	fp = fdopen(fd, mode);
 #endif
-	PyMem_FREE(mode);
 	Py_END_ALLOW_THREADS
+	PyMem_FREE(mode);
 	if (fp == NULL)
 		return posix_error();
 	f = PyFile_FromFile(fp, "<fdopen>", orgmode, fclose);