Add py.test to the list of tested pytest names

OpenBSD changed the name of the pytest script from py.test-3 in OpenBSD 6.8 to py.test in OpenBSD 6.9.

The py.test-3 name which was added in d5562a3e for the sake of OpenBSD and CentOS is still required for CentOS.

Closes #2693 (closed)