Make Jsproxy getattr and dir use GenericGetAttr and object.__dir__

[hoodmane]

Before this, the JsProxy getattr method explicitly compared the attribute name against a list of strings to decide whether to pull the attribute off of a python object. dir only returned the keys of the wrapped javascript object, not the extra keys coming from python.

Now I use PyObject_GenericGetAttr so that if JsProxy.__dict__ has a key, we return the associated value, and dir returns the merged list of python and javascript keys.

This shadows more attributes of the javascript object, but they are all names with __underscores__ so I think it's okay.

[hoodmane]

Similar changes are included in #972 but I figured this was worth separating off.

[rth]

Now I use PyObject_GenericGetAttr so that if JsProxy.dict has a key, we return the associated value, and dir returns the merged list of python and javascript keys.

Might be good to add a test for it?

Also could you rename this PR to something more specific than "redo"?

[rth]

We should add a changelog entry, otherwise LGTM.

[hoodmane]

Okay updated changelog. A little hard to describe because this is more of a backend thing, but it does have useful visible effects on the user like typeof and new now appear in dir(jsproxy).