py: Make gc.enable/disable just control auto-GC; alloc is still allowed.

gc.enable/disable are now the same as CPython: they just control whether
automatic garbage collection is enabled or not.  If disabled, you can
still allocate heap memory, and initiate a manual collection.
diff --git a/py/objexcept.c b/py/objexcept.c
index 9b39788..4b8d6ea 100644
--- a/py/objexcept.c
+++ b/py/objexcept.c
@@ -28,6 +28,7 @@
 #include <stdarg.h>
 #include <assert.h>
 #include <stdio.h>
+#include <stdint.h>
 
 #include "mpconfig.h"
 #include "nlr.h"