handle dunder names in @declared_attr separately

handle dunder names in @declared_attr separately

Fixed Mypy crash which would occur when using Mypy plugin against code
which made use of :class:`_orm.declared_attr` methods for non-mapped names
like ``__mapper_args__``, ``__table_args__``, or other dunder names, as the
plugin would try to interpret these as mapped attributes which would then
be later mis-handled. As part of this change, the decorated function is
still converted by the plugin into a generic assignment statement (e.g.
``__mapper_args__: Any``) so that the argument signature can continue to be
annotated in the same way one would for any other ``@classmethod`` without
Mypy complaining about the wrong argument type for a method that isn't
explicitly ``@classmethod``.

Fixes: #7321
Change-Id: I55656e867876677c5c55143449db371344be8600