Define pytest_plugins at the root to support pytest 4.0

```
Defining 'pytest_plugins' in a non-top-level conftest is no longer supported:
It affects the entire test suite instead of just below the conftest as expected.
  /home/dev/swh-environment/swh-web/swh/web/tests/conftest.py
Please move it to a top level conftest file at the rootdir:
  /home/dev/swh-environment/swh-web
For more information, visit:
  https://docs.pytest.org/en/stable/deprecations.html#pytest-plugins-in-non-top-level-conftest-files
```

conftest.py

+pytest_plugins = ["swh.auth.pytest_plugin"]

swh/web/tests/conftest.py

@@ -25,8 +25,6 @@ from swh.web.common.typing import OriginVisitInfo
 from swh.web.config import get_config
 from swh.web.tests.data import get_tests_data, override_storages
 
-pytest_plugins = ["swh.auth.pytest_plugin"]
-
 # Used to skip some tests
 ctags_json_missing = (
     shutil.which("ctags") is None