#18389: Clarify that relpath does not access the file system.

Initial patch by Madison May.
Issue #18365: Add mock Text class and test thereof versus tk.Text.
Based on patches by Todd.Rovito and Phil Webster.
Issue #18365: normalize whitespace
#18437: fix comment typo.
Issue #18279: Add tests for idlelib/RstripExtension.py. Original patch by
Phil Webster. With that available, modify RstripExtension.py to stop deleting
null slices, which caused a file to be marked as changed when it was not.
Issue #18365: convert buildbot errors to skips.