Set asyncio WindowsSelectorEventLoopPolicy on Windows

tests/conftest.py

@@ -1,4 +1,6 @@
+import asyncio
 import logging
+import os
 from math import inf
 from threading import Event
 from typing import Any, Callable, no_type_check
@@ -40,6 +42,11 @@
     resource.setrlimit(resource.RLIMIT_NOFILE, (soft, hard))
 
 
+# Enforce selector event loop on Windows.
+if os.name == "nt":
+    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())  # type:ignore
+
+
 class TestSession(Session):
     """A session that copies sent messages to an internal stream, so that
     they can be accessed later.