handle case when no event loop exists (support pytest-aiohttp)

Author: smagafurov

pytest_asyncio/plugin.py

@@ -129,7 +129,12 @@ async def setup():
             if kw not in request.keywords:
                 continue
             policy = asyncio.get_event_loop_policy()
-            old_loop = policy.get_event_loop()
+            try:
+                old_loop = policy.get_event_loop()
+            except RuntimeError as exc:
+                if 'no current event loop' not in str(exc):
+                    raise
+                old_loop = None
             policy.set_event_loop(loop)
             fixturedef.addfinalizer(lambda: policy.set_event_loop(old_loop))