Shoobx.junitxml

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- Nothing changed yet.

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- Rename command line option `--xml`` to ``--junitxml`` since zope.testrunner 6.3.0
added the same option and there's a conlict

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- Fixed documentation.

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- Support for latest zope.testrunner and other latest dependencies
- Added missing stderr and stdout params to test_failure and test_error
methods of XMLOutputFormattingWrapper
(prevented xml report file generation in zope.testrunner >= 5.1)
- dropped support for python 2, updated support for 3.X

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- Add support for Python 3.7

- Add support for the latest `zope.testrunner`, which switches from `optparse`
to `argparse`.

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- Record doctest cases in modules where test cases are defined,
instead of putting them all in 'doctest' module.