tests.generate: stop using asyncio.get_event_loop (#349)

The use of `asyncio.get_event_loop()` has been deprecated in python 3.10+. We replace this usage with `asyncio.run()` for python 3.7+.

tests/generate.py

@@ -159,8 +159,18 @@ def main():
         whitelist = set(sys.argv[1:])
 
     if platform.system() == "Windows":
-        asyncio.set_event_loop(asyncio.ProactorEventLoop())
+        # for python version prior to 3.8, loop policy needs to be set explicitly
+        # https://docs.python.org/3/library/asyncio-policy.html#asyncio.DefaultEventLoopPolicy
+        try:
+            asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
+        except AttributeError:
+            # python < 3.7 does not have asyncio.WindowsProactorEventLoopPolicy
+            asyncio.get_event_loop_policy().set_event_loop(asyncio.ProactorEventLoop())
 
+    try:
+        asyncio.run(generate(whitelist, verbose))
+    except AttributeError:
+        # compatibility code for python < 3.7
         asyncio.get_event_loop().run_until_complete(generate(whitelist, verbose))