Assertionlib

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* Added `assertion.issuperset()` and `assertion.issubset()`.
* Export the source to PyPi.
* Updated MyPy settings.

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* Added `Nano-Utils <https://github.com/nlesc-nano/Nano-Utils>`_ as a dependency;
moved a number of functions there.
* Removed `requirements.txt` in favor of `.readthedocs.yml`.
* Removed travis tests in favor of GitHub Actions.
* Removed `CITATION.cff` in favor of Zenodo.

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* Added codecov and zenodo badges.

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* Added support for the ``python stup.py test`` command.
* Added support for the ``python stup.py build_sphinx`` command.
* Added a `GitHub Actions <https://github.com/features/actions>`_ workflow for automatic PyPi publishing.
* Enabled `codecov <https://codecov.io/>`_ for the GitHub Action tests.
* Updated the package classifiers and keywords in ``setup.py``.

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* Added the ``AssertionManager.xor()``, ``AssertionManager.isdisjoint()`` and ``AssertionManager.length_hint()`` methods.
* Annotate most ``AssertionManager`` methods using Protocols.
* Moved Protocols to their own separate stub module.
* Cleaned up the ``_MetaAM`` metaclass.
* Reworked some of the internals of ``AssertionManager``.
* Added tests using `pydocstyle <https://github.com/henry0312/pytest-pydocstyle>`_.

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* Windows bug fix: Check for the presence of the ``AssertionManager._isdir()``
rather than the type of operating system.