try_rich_to_3way_compare is documented as returning -2 
for an exception.  However, in the loop, when 
try_rich_compare_bool returns -1, 
try_rich_to_3way_compare returns -1 instead of -2.  
try_rich_compare_bool does return -1 in case of 
exception.

Unclear whether this is a bug in the code or the 
comment.  The only caller of try_rich_to_3way_compare 
is do_cmp, and the latter's return conditions aren't 
documented, although at least one of do_cmp's callers 
documents its own return conditions as "-2 for an 
exception" too.

So best guess is that the code is in error, and 
that "return -1" should be replaced by "return -2" in 
try_rich_to_3way_compare.