py: Declare constant data as properly constant.

Otherwise some compilers (eg without optimisation) will put this read-only
data in RAM instead of ROM.
diff --git a/py/parse.c b/py/parse.c
index 0b60569..7da484c 100644
--- a/py/parse.c
+++ b/py/parse.c
@@ -104,7 +104,7 @@
 #undef one_or_more
 #undef DEF_RULE
 
-STATIC const rule_t *rules[] = {
+STATIC const rule_t *const rules[] = {
 #define DEF_RULE(rule, comp, kind, ...) &rule_##rule,
 #include "py/grammar.h"
 #undef DEF_RULE