py: Make mp_obj_get_type() return a const ptr to mp_obj_type_t.

Most types are in rodata/ROM, and mp_obj_base_t.type is a constant pointer,
so enforce this const-ness throughout the code base.  If a type ever needs
to be modified (eg a user type) then a simple cast can be used.
diff --git a/py/sequence.c b/py/sequence.c
index 15e9250..94a9bb9 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -183,7 +183,7 @@
 
 // Special-case of index() which searches for mp_obj_t
 mp_obj_t mp_seq_index_obj(const mp_obj_t *items, size_t len, size_t n_args, const mp_obj_t *args) {
-    mp_obj_type_t *type = mp_obj_get_type(args[0]);
+    const mp_obj_type_t *type = mp_obj_get_type(args[0]);
     mp_obj_t value = args[1];
     size_t start = 0;
     size_t stop = len;