Close #19025: Better error message when trying to delete an Enum member.

Also slight code reorg for PEP 8 guidelines.
Issue #19034: repr() for tkinter.Tcl_Obj now exposes string reperesentation.
Issue #18996: TestCase.assertEqual() now more cleverly shorten differing
strings in error report.
Issue #19028: Fixed tkinter.Tkapp.merge() for non-string arguments.
Null merge (tkinter.Tkapp.merge() was removed in 3.4).