modernize and repair inheritance examples

modernize and repair inheritance examples

remarkably, the examples for concrete and single were still
using classical mappings.    Ensure all three examples use
modern declarative patterns, each illustrate the identical set
of query operations.  Use back_populates, flat=True for joins,
etc. ensure flake8 linting, correct links and add a link back
from newly reworked inheritance documentation.

Change-Id: I8465a9badbb0eda804f457ccac599f051ee3c27c
(cherry picked from commit 5329fc481ed1dcea0e922fd63def3094bda12c91)