devel/py-pytest-{pep8,translations}: switch to pytest4

These plugins are not compatible with recent pytest.

Consumers (sysutils/iocage and www/py-flask-security) were switched
to pytest4 as well. Note that py-flask-security tests were and still
are broken due to unrelated problem - py-sqlalchemy12 and
py-sqlalchemy14 conflict.

PR: 256624