py: Give up and make mp_obj_str_get_data() deal with bytes too.

This is not fully correct re: error handling, because we should check that
that types are used consistently (only str's or only bytes), but magically
makes lot of functions support bytes.

tests/basics/int1.py

@@ -46,6 +46,7 @@ print(int('0B100', 2))
 print(int('0100', 2))
 print(int(' \t 0o12', 8))
 print(int('0o12  \t  ', 8))
+print(int(b"12", 10))
 
 
 def test(value, base):