assert.h was not always included by Python.h; make sure we import it for

older versions.  (Thanks to Martijn Faassen.)

Modules/pyexpat.c

@@ -1,4 +1,7 @@
 #include "Python.h"
+#if PY_VERSION_HEX < 0x020000B1
+#include <assert.h>
+#endif
 #include <ctype.h>
 
 #include "compile.h"