Z3c-insist

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- Dropped support for Python 3.6
- Added support for Python 3.9
- Applied zopefoundation.meta config

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- Dropped support for Python 2 and Python 3.5
- Added support for Python 3.8

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- Use `ConfigParser.read_file()` instead of `ConfigParser.readfp()`, since the
latter is deprecated.

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- Fixed a senseless bug that was caused by the Py3 port.

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- Fixed a bug in reading the config files.

- Switched to modern ``io.open()`` which supports the ``encoding``
parameter.

- Make sure that Bytes are properly converted in both directions.

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- Ensure that we always load files in a way that its content is automatically
converted to unicode.

- Ensure that MANIFEST is complete.