gh-113317: Change how Argument Clinic lists converters (#116853)

* Add a new create_parser_namespace() function for
  PythonParser to pass objects to executed code.
* In run_clinic(), list converters using 'converters' and
  'return_converters' dictionarties.
* test_clinic: add 'object()' return converter.
* Use also create_parser_namespace() in eval_ast_expr().

Co-authored-by: Erlend E. Aasland <erlend@python.org>