Issue #6181: Fixed errors in tkinter.Listbox docstrings.
Based on patch by Guilherme Polo.
Issue #6181: Fixed minor bugs in tkinter.Listbox methods:
bbox(), curselection() and get().
Issue #18492: Allow all resources when tests are not run by regrtest.py.

This changeset also includes cleanup allowed by this behavior change.
Issue 21533: Dicts take iterables, not iterators. Patch by Wolfgang Maier.