Rework AliasedClass __getattr__ to use top-level getattr()

Rework AliasedClass __getattr__ to use top-level getattr()

Reworked the attribute mechanics used by :class:`.AliasedClass` to no
longer rely upon calling ``__getattribute__`` on the MRO of the wrapped
class, and to instead resolve the attribute normally on the wrapped class
using getattr(), and then unwrap/adapt that.  This allows a greater range
of attribute styles on the mapped class including special ``__getattr__()``
schemes; but it also makes the code simpler and more resilient in general.

Fixes: #4694
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: I28901e2472d3c21e881fe5cafa3b1d3af704fad8