Pluginlib

- 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

- [PluginLoader.plugins_all](https://pluginlib.readthedocs.io/en/stable/api.html#pluginlib.PluginLoader.plugins_all) now returns an [OrderedDict](https://docs.python.org/3/library/collections.html#collections.OrderedDict) at the lowest level.
- The newest plugin version is last
- Abstract coroutine methods are now checked for in child classes
- If [async](https://docs.python.org/3/library/asyncio-task.html#coroutines) is supplied on the parent method, it must be supplied on the child method
- FAQ updated
- [Why does calling `super()` with no arguments in a parent class raise a `TypeError`?](https://pluginlib.readthedocs.io/en/stable/faq.html#why-does-calling-super-with-no-arguments-in-a-parent-class-raise-a-typeerror)
- [Why am I getting `TypeError: metaclass conflict`?](https://pluginlib.readthedocs.io/en/stable/faq.html#why-am-i-getting-typeerror-metaclass-conflict)

- Minor fixes for linting
- Correct import of collections.abc.Iterable
- Change PyPI status from Beta to Stable

- Added empty `__slots__` to Plugin class to support children without `__dict__`
- Support `__slots__` in parent classes

- Added [PluginLoader.plugins_all](https://pluginlib.readthedocs.io/en/latest/api.html#pluginlib.PluginLoader.plugins_all) property
- Added [PluginLoader.get_plugin()](https://pluginlib.readthedocs.io/en/latest/api.html#pluginlib.PluginLoader.get_plugin) method
- Added plugin [type filters](https://pluginlib.readthedocs.io/en/latest/concepts.html#type-filters)