Fix compat issue with older SQLAlchemy versions.

Fix compat issue with older SQLAlchemy versions.

Added placeholder classes for ``Computed`` and ``Identity`` when older 1.x
SQLAlchemy versions are in use, namely prior to SQLAlchemy 1.3.11 when the
``Computed`` construct was introduced. Previously these were set to None,
however this could cause issues with certain codepaths that were using
``isinstance()`` such as one within "batch mode".

Fixes: #1237
Change-Id: I033712158baa68cca6a56bd41d0636109e156b86