Wagtail 5 Support

Main motivation for this new release was to support 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

Compatibility with newer Wagtail versions

Replaced poetry with flit for the package build process.

Features

• Compatibility with newer Wagtail versions
  • #2 issue by @djch

Refactoring

• Rename master -> main
  • #3 issue by @ephes

Fixes

• Make tox work again
  • #2 issue by @ephes