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.

py/qstr.c

@@ -26,6 +26,7 @@
 
 #include <assert.h>
 #include <string.h>
+#include <stdint.h>
 
 #include "mpconfig.h"
 #include "misc.h"