Pluginlib

- Non-functional bug fixes

- Add support for Python 3.12

- Drop Python 2.6 support
- Minor fix to enable Python 3.11-dev support
- Fix issue where module attributes weren't accessible when importing by path
- Fix issue where modules were imported multiple times when importing by path on Python 3.5+
- Switched to GitHub Actions for testing
- Exclude test bytecode from dist tarball
- Miscellaneous minor tweaks and fixes

- Fix tests for 3.10-dev

- Minor cleanup
- Fix tests for 3.10-dev

- Prevent double throwing exception in Python 3
- Import paths no longer require `__init.py__` files to import recursively
- Type annotations are now only compared when both parent and child methods are annotated
- New [plugin_type](https://pluginlib.readthedocs.io/en/stable/api.html#pluginlib.Plugin.plugin_type) property
- New [plugin_group](https://pluginlib.readthedocs.io/en/stable/api.html#pluginlib.Plugin.plugin_group) property
- Refactored class inspection