mypy plugin fixes

mypy plugin fixes

Adjustments made to the mypy plugin to accommodate for some potential
changes being made for issue #236 sqlalchemy2-stubs when using SQLAlchemy
1.4. These changes are being kept in sync within SQLAlchemy 2.0.
The changes are also backwards compatible with older versions of
sqlalchemy2-stubs.

Fixed crash in mypy plugin which could occur on both 1.4 and 2.0 versions
if a decorator for the :func:`_orm.registry.mapped` decorator were used
that was referenced in an expression with more than two components (e.g.
``@Backend.mapper_registry.mapped``). This scenario is now ignored; when
using the plugin, the decorator expression needs to be two components (i.e.
``@reg.mapped``).

References: https://github.com/sqlalchemy/sqlalchemy2-stubs/issues/236
Fixes: #9102
Change-Id: Ieb1bf7bf8184645bcd43253e57f1c267b2640537