Improve class loading

[dmamelin]

Improved class loading: added support for dataclass, enum, metaclass, and keywords, and now ignoring inaccessible attributes during class creation (seen when instantiating homeassistant.helpers.entity.Entity due to ABCCachedProperties attribute caching).
Fixes #142, #537, #603.

[ALERTua]

it would be nice for the PR to contain only one major change for the diff readability :)
are you sure the tests cover every new supported concept?
will try this PR on my production.

[ALERTua]

I have found no problems while using this PR. I didn't yet try the new supported concepts though.

[dmamelin]

@ALERTua
Yes, the tests don’t cover all new features. The primary goal was to improve native class loading, especially for dataclass and enum - and that part is covered by tests.

[ALERTua]

if you would be so kind to also expand the documentation <3

[dmamelin]

if you would be so kind to also expand the documentation <3

Which part exactly?
I think the language-limitations section already describes the capabilities and restrictions fairly well.
"The language coverage is relatively complete" - we just need to move closer to complete.
There isn’t a separate section listing all supported features.

[craigbarratt]

Thanks! Classes in pyscript are definitely a weak area in terms of completeness and correctness, so these improvements are much appreciated. Thanks for adding some tests.