The attached patch ports the implementation of keyword-only arguments
from revision 52491 back to trunk.

This is the first time I've worked on the C internals, so here are some
notes:

- test_collections is the only test which fails, because it tries to
call a function with more than 255 arguments, which results in a
SyntaxError because of the following added code in Python/ast.c. What
should be done about it?

    if (nposargs + nkwonlyargs > 255) {
        ast_error(n, "more than 255 arguments");
        return NULL;
    }

- The patch only adds what's in revision 52491. There is at least one
more change involving keyword-only arguments, for example issue1573. Are
there others? Should they be included in this patch or in a separate one?

- There are some changes which were generated, like Python/Python-ast.c
(which needs to be checked in separately).

- Is there documentation which needs to be updated?