Wagtail-srcset

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* Remove stale test images after test run via autouse fixture closes 4

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* Add support for Wagtail>=5
* Drop support for Wagtail<3.0
* Added workaround for image_node.filter_spec -> image_node.filter_specs change in Wagtail 5
* Some small spelling fixes and extended docstrings
* pre-commit autoupdate
* make tox check against python (3.9, 3.10, 3.11) and wagtail (3, 4, 5)
* fixed codecov.io workflow
* added supported python and wagtail versions to readme via badges

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* Replace poetry with flit
* Make tox run tests with flit for last 3 Python versions
* Updated dependencies

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* Bumped down python requirement to 3.6 (since f-strings are used) - requeste by CVYY39
* Updated dependencies

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* Updated dependencies

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* Add original url to rendition generated by "as" syntax