Snap-helpers

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- Rebuild to fix version

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- Add missing py.typed marker to package
- [docs] Fix ReadTheDocs setup

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- Add support for ``snapctl refresh``.
- Add support for ``snapctl system-mode``.
- Add option to make fail ``write-hooks`` fail when no hook is found.
- Drop support for Python 3.6 and ``core18``. (13)
- Replace ``pkg_resources`` with ``importlib.metadata``. (16)
- Rework project setup, move to ``pyproject.toml`` only.
- [gh] Switch to Codecov action.

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- Add ``SnapConfig.unset``.
- Rework the test snap and Add integration tests.
- [gh] Add actions for running integration tests against supported bases.

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- Replace shell script for hooks, direcly generate Python scripts instead.
- Add ``SnapPaths.real_home`` path.
- [snap] Enable colors in IPython configuration.

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- Add support in ``SnapCtl`` for interacting with plugs and slots.
- Support ``core22``-based snaps.
- [snap] Rebase snap on ``core22``.
- Add integration tests.