Cleaned up and speed up Transience tests a bit. Now they take 20 seconds out of 50 seconds total for Zope2 - before it was 30 out of 60. There should still be some more room for improvement.

Even more updates to deal with the doctest module, including the forward port of c111722 from the 2.12 branch

- removed obsolete code that did support string exceptions
- some related cleanup

Remove change entries for changes already found in 2.12. No need for duplication until we hit a 2.13.0 final release

Updated to the latest zope.testing and backport the deprecation avoidance changes for zope.testing.doctest