Fix: changed size during iteration when pretty-printing dicts and sets - #15469

Fixes #14541

The dict and set printers looped over the live container while printing each
item. Printing an item can change the container: for vars(posix), printing
posix.chmod calls inspect.signature(), which runs eval(..., posix.__dict__),
and eval adds __builtins__ to that dict. Sets with at least max_seq_length
items are not sorted first, so they had the same problem
(Set changed size during iteration).

RepresentationPrinter._enumerate now loops over a copy of at most
max_seq_length + 1 items, which fixes both printers and avoids copying the
whole of a large dict that will be cut off anyway.