py: Small changes to objstr.c, including a bug fix.

Some small fixed:

- Combine 'x' and 'X' cases in str format code.

- Remove trailing spaces from some lines.

- Make exception messages consistently begin with lower case (then
needed to change those in objarray and objtuple so the same
constant string data could be used).

- Fix bug with exception message having %c instead of %%c.
diff --git a/py/objtuple.c b/py/objtuple.c
index fc97f53..2be0440 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -166,7 +166,7 @@
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(self->len, index, &slice)) {
                 nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
-                    "Only slices with step=1 (aka None) are supported"));
+                    "only slices with step=1 (aka None) are supported"));
             }
             mp_obj_tuple_t *res = mp_obj_new_tuple(slice.stop - slice.start, NULL);
             mp_seq_copy(res->items, self->items + slice.start, res->len, mp_obj_t);