Py-html-checker

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Fix issue with currently used Jinja version (2.x) which did not pinned MarkupSafe
version but use a ``soft_unicode`` function which have been dropped since
MarkupSafe 2.1.0. This leaded to HTML export to be unavailable even with Jinja

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* Include a fix proposed by acbaraka to enhance Window support, even it's not an
official support;
* Clean validator output from a line information about environment variable
``_JAVA_OPTIONS``, close 22;
* Add Tox to dev requirements;
* Remove Python 3.5 support;
* Validate support for Python 3.6 to 3.8;

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* Add this changelog;
* Upgrade to ``vnu==20.6.30``;
* Rename commandline name from ``html-checker`` to ``htmlcheck`` following this
document `<https://smallstep.com/blog/the-poetics-of-cli-command-names/>`_;
* Add ``twine`` in extra requirements ``dev`` and use it in for Makefile
``release`` action;

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First usable version with HTML and JSON exporters and every required vnu option
support.