Shamelessly cloned from the parallel code in pltcl, an exception for
void in denying pseudotypes being returned. Pl/tcl didn't reference
VOIDOID anywhere else, so ... .
Allowed following trivial test function to succeed:
create or replace function set_gd(int) returns void as
$$
GD['global_count'] = args[0]
$$ language plpythonu volatile;