pytest_plugins in root conftest.py not working with tox

[fschulze]

Hi!

In devpi-server are several fixtures which are reused in plugin tests. That isn't optimal, see devpi/devpi#780. Some other fixtures are currently reused by importing them from test_devpi_server.conftest, some other with pytest_devpi_server which is a plugin with an entry point. The direct importing also seems to be bad for various reasons.

After trying for some time I managed to get it working by using pytest_plugins = ["pytest_devpi_server", "test_devpi_server.conftest"] in the root conftest.py of devpi-web.

Problem is, it only works when using pytest directly, not when using tox. I tried to debug it and am stuck. The modules are reached, which is confirmed by both tox -e py37 -- --trace-config and sprinkling pdb.set_trace() in them. All relevant modules pytest_devpi_server, test_devpi_server.conftest, web/conftest.py and web/tests/conftest.py are reached and included in the trace. Still the fixtures from test_devpi_server.conftest are not found when running tox, but work fine when using pytest directly.