Add slots to core EventBus

[bdraco]

Proposed change

Use slots in the EventBus

Fix all the tests that were patching the EventBus (and firing state change events with invalid entity ids)

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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Additional information

• This PR fixes or closes issue: fixes #
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• Link to documentation pull request:

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[bdraco]

It looks like quite a few integrations patch listen.

[bdraco]

Will save this one for the next long flight as it will be a bit of a grind to refactor the tests

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[MartinHjelmare]

Thanks!

[bdraco]

Thanks