* Fix py::kw_only when used before the first arg of a method
The implicit space for the `self` argument isn't added until we hit the
first argument, but this wasn't being done for kw_only or pos_only, and
so a kw_only before the first argument would break.
This fixes it by properly checking whether we need to add the self arg.
(The pos_only issue here was extremely mild -- you didn't get the `/` in
the docstring, but AFAICT it has no other effect since there are no
meaningful arguments before it anyway).
* Style changes
- rename check_have_self_arg -> append_self_arg_if_needed
- move the argument name inline comments before the args instead of
after
