restore inheritance-diagram commands to docs

[mscarey]

I deleted the inheritance-diagram commands from the docs because ReadTheDocs failed to render them, and just showed the graphviz text. For example:

digraph inheritance0e43d7d153 { bgcolor=transparent; rankdir=LR; size="8.0, 12.0"; "ABC" [fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica, sans",fontsize=10,height=0.25,shape=box,style="setlinewidth(0.5),filled",tooltip="Helper class that provides a standard way to create an ABC using"]; "Assertion" [URL="#nettlesome.statements.Assertion",fillcolor=white,fontname="Vera Sans, DejaVu Sans, Liberation Sans, Arial, Helvetica,

It gave the error message WARNING: dot command ‘dot’ cannot be run (needed for graphviz output), check the graphviz_dot setting

The issue is the subject of an unanswered StackOverflow question:
See https://stackoverflow.com/questions/63503855/warning-dot-command-dot-cannot-be-run#

I have a feeling the problem would go away if I switched from setup.cfg to setup.py