Handle abstract properties

[Eric-Arellano]

Part of #2330.

FYI: attributes don't have modifiers

Our old code would try to remove modifiers for py.attribute, but that was not necessary because they cannot be set. (An attribute is the default, like setting a value in __init__; a property is when you set @property on a function)

Modifiers are only set on properties. This is evidenced by npm run regen-api not changing despite removing code to handle py.attribute.

PR does not preserve property modifier

For a typical end-user, an attribute and property are essentially the same. The only time I think it'd be relevant is if you're subclassing the class; otherwise, I think property is distracting.

So, we continue to not preserve property, the same as before.

abstract vs abstract property

We capture the whole abstract property because the phrase is much more meaningful than abstract.

[Eric-Arellano]

This is safe to merge even before we teach closed source to handle modifiers. The app will simply ignore the value.

[frankharkins]

Nice improvement, thank you!