Allow passing multiple events to the @listen decorator

[Jonxslays]

Summary

This pull requests allows passing multiple events to the listen decorator.
Alternatively, a Union can be used in the type hint to achieve the same thing.

@bot.listen(hikari.GuildAvailableEvent, hikari.GuildJoinEvent)
async def on_guild_available(event: ...) -> None:
    ...

@bot.listen()
async def on_start(event: hikari.StartingEvent | hikari.StartedEvent) -> None:
    ...

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Flake8 session is failing - but it was on master before I touched anything.

[thesadru]

Ideally, the origin of the annotation should be checked too in case a type with __args__ is passed in.
Note that in 3.9 there's only typing.Union but in 3.10 you must also check for types.UnionType

[davfsa]

Ideally, the origin of the annotation should be checked too in case a type with __args__ is passed in.
Note that in 3.9 there's only typing.Union but in 3.10 you must also check for types.UnionType

This works with all python versions we support, as it never actually checks for the union. It just expands the arguments of the annotation using a typing function, which will support each python version

[thesadru]

Oops, I wanted to suggest also checking the origin for the edge cases when the passed-in type is not a union. 😅

[davfsa]

Oops, I wanted to suggest also checking the origin for the edge cases when the passed-in type is not a union. sweat_smile

In the case it is not a union, it will get expanded as far as it can go and then passed onto subscribe. Subscribe will then make the necessary checks and error accordingly

[FasterSpeeding]

Also this needs a changelog entry

[FasterSpeeding]

Linting is failing

[davfsa]

Lining failing due to psf/black#2964

Will be fixed by updating black to the latest version, which I will open a PR for in a sec. Then this PR should be rebased

Relevant PR: #1114