Fix: @properties and categorize them as attributes

[pedrolabonia]

Fixes #14, Fixes #9

We classify if functions are a @property by checking if they have property in labels.

Fixes:

• @property now shows correctly alongside the attributes
• @property renders correctly with CSS class doc-attribute
• @property doesn't show parens anymore
• All objects are not properly categorized (see issue#14)

For material theme only for now

[pedrolabonia]

Oh, I still need to check for property here:

https://github.com/pedrolabonia/python/blob/d2d0168af31bd0aff05202cdff464476b2d9dfad/src/mkdocstrings_handlers/python/templates/material/_base/children.html#L90

I'll commit that tomorrow

[pawamoy]

Please keep separated changes in different PRs 🙂
One for #9 first. #14 needs something different (see my comment there). And the properties re-categorization should happen in a third PR, but I'd like that we discuss it a bit more, as it could be something we do in Griffe directly (see comment in #9).