Configure the async loop at test session start

Doing it in a fixture is too late: event_loop might have already been
called if the first test running is async, and it would fail on Windows.

This started to be needed after pytest-asyncio 0.17.0 was released.
See <pytest-dev/pytest-asyncio#256>.

Author: dvarrazzo

tests/conftest.py

@@ -62,24 +62,15 @@ def retries(request):
         )
 
 
-@pytest.fixture
-def event_loop(request):
-    """Return the event loop to test asyncio-marked tests."""
-    # pytest-asyncio reset the the loop config after each test, so set
-    # set them each time
-
-    loop = request.config.getoption("--loop")
+def pytest_sessionstart(session):
+    # Configure the async loop.
+    loop = session.config.getoption("--loop")
     if loop == "uvloop":
         import uvloop
 
         uvloop.install()
     else:
         assert loop == "default"
 
-    loop = None
     if sys.platform == "win32":
         asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
-    if not loop:
-        loop = asyncio.get_event_loop_policy().new_event_loop()
-    yield loop
-    loop.close()