adapt pytest plugin to support pytest v7

adapt pytest plugin to support pytest v7

Implemented support for the test suite to run correctly under Pytest 7.
Previously, only Pytest 6.x was supported for Python 3, however the version
was not pinned on the upper bound in tox.ini. Pytest is not pinned in
tox.ini to be lower than version 8 so that SQLAlchemy versions released
with the current codebase will be able to be tested under tox without
changes to the environment.   Much thanks to the Pytest developers for
their help with this issue.

Change-Id: I3b12166199be2b913ee16e78b3ebbff415654396