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Bugfix release:
* Change return value from math_dollar doctest parser to be list of strings,
one per line, rather than a single multiline string. This makes it possible
for other ReST transforms to run correctly on the output, such as links and
substitutions. Add tests to confirm this works correctly.
* Add ability to import ``math_dollar`` and ``mathcode`` from top level import
of ``texext``.
* Refactor testing to use ``sphinxtesters`` module.