Zope-testing

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- Remove import of ``zope.testing.doctest`` from ``zope.testing.renormalizer``.

- Suppress output to ``sys.stderr`` in ``testrunner-layers-ntd.txt``.

- Suppress ``zope.testing.doctest`` deprecation warning when running
our own test suite.

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- Fix broken ``from zope.testing.doctest import *``

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- No changes; reupload to fix broken 3.9.0 release on PyPI.

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- Modify the testrunner to use the standard Python ``doctest`` module instead
of the deprecated ``zope.testing.doctest``.

- Fix ``testrunner-leaks.txt`` to use the ``run_internal`` helper, so that
``sys.exit`` isn't triggered during the test run.

- Add support for conditionally using a subunit-based output
formatter upon request if subunit and testtools are available. Patch
contributed by Jonathan Lange.

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- Downgrade the ``zope.testing.doctest`` deprecation warning into a
PendingDeprecationWarning.

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- Add ``MANIFEST.in`` and reupload to fix broken 3.8.5 release on PyPI.