Move pytest_plugin declaration to top-level conftest

Defining 'pytest_plugins' in a non-top-level conftest is no longer supported
because it affects the entire directory tree in a non-explicit way.

conftest.py

@@ -18,3 +18,8 @@ def swhmain():
     yield _swhmain
     _swhmain.commands = commands
     _swhmain.aliases = aliases
+
+
+# This is coming from the aiohttp library directly. Beware the desynchronized
+# https://github.com/aio-libs/pytest-aiohttp module which wraps that library...
+pytest_plugins = ["aiohttp.pytest_plugin"]

swh/core/api/tests/conftest.py

-# This is coming from the aiohttp library directly. Beware the desynchronized
-# https://github.com/aio-libs/pytest-aiohttp module which wraps that library...
-pytest_plugins = ["aiohttp.pytest_plugin"]