fix mpy magic numbers

py/persistentcode.c

@@ -591,12 +591,12 @@ STATIC void save_raw_code(mp_print_t *print, const mp_raw_code_t *rc) {
 
 void mp_raw_code_save(mp_compiled_module_t *cm, mp_print_t *print) {
     // header contains:
-    //  byte  'M'
+    //  byte  'C' (CIRCUITPY)
     //  byte  version
     //  byte  native arch (and sub-version if native)
     //  byte  number of bits in a small int
     byte header[4] = {
-        'M',
+        'C',
         MPY_VERSION,
         cm->has_native ? MPY_FEATURE_ENCODE_SUB_VERSION(MPY_SUB_VERSION) | MPY_FEATURE_ENCODE_ARCH(MPY_FEATURE_ARCH_DYNAMIC) : 0,
         #if MICROPY_DYNAMIC_COMPILER