Convert Python types to proper Python type hierarchy.

Now much more inline with how CPython does types.
diff --git a/py/objstr.c b/py/objstr.c
index 03a7618..27c9440 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -179,6 +179,7 @@
     { &mp_const_type },
     "str",
     str_print, // print
+    NULL, // make_new
     NULL, // call_n
     NULL, // unary_op
     str_binary_op, // binary_op