[swappable] Annotate decorator with original type

[spencerp]

Patch description
Unfortunately Python doesn't support an Intersection type hint (though they've been talking about it since 2014: https://www.python.org/dev/peps/pep-0483/).

So we have to choose whether we tell static type checkers that the class returned by @swappable is the original class or a class that conforms to ModularComponent, we can't communicate both.

I think it makes more sense to type check agains the original class, since it's much more likely that people will be accessing properties and methods of the original class rather than swappables and ModularSubcomponents, which are meant to be somewhat invisible in most cases.

Testing steps
Open up IDE and verify that calls to properties of @swappable classes don't throw static type checking errors.
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