Sphinx-hoverxref

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* Improve intersphinx robustness
* Update all dependencies for building documentation
* Packaging: use bumpver and flit to build/publish
* Improve documentation installing instructions
* Setup pip-tools together with dependabot to handle dependencies updates
* Remove tests for Python 3.6 and Python 3.7 and add newer Sphinx versions (4.4, 4.5 and 5.0)

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* Ajax header: send the header on each call instead with .ajaxSetup (https://github.com/readthedocs/sphinx-hoverxref/pull/167)
* Tests: solve problem with RTD documentation's title (https://github.com/readthedocs/sphinx-hoverxref/pull/168)

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* Official release

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* Send ``X-HoverXRef-Version`` HTTP header when hitting the API

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* Move from TravisCI to CircleCI
* Use Read the Docs' Embed APIv3
* Stop adding ``data-`` attributes to HTML nodes
* Do not require ``hoverxref_project`` and ``hoverxref_version`` defined anymore
* Simplify code by removing ``HoverXRefHTMLTranslatorMixin`` override required and utils functions to get xref data
* Add new config ``hoverxref_sphinx_version`` used to send to Embed APIv3
* Update all Python requirements
* Remove via Javascript the ``title=`` property on Intersphinx references
* Support for ``glossary``/``term`` added
* Support for ``sphinxcontrib-bibtex`` added
* Improve documentation to show examples of documentation not hosted on Read the Docs
* Use regular expressions on tests to allow partial matching when required