tornado: construct AsyncHTTPClient each time so that it uses the right ioloop

This is a better fix for #129.

nvchecker/source/tornado_httpclient.py

@@ -17,7 +17,6 @@ from .httpclient import DEFAULT_USER_AGENT
 
 __all__ = ['session', 'HTTPError', 'NetworkErrors']
 
-client = AsyncHTTPClient()
 HTTP2_AVAILABLE = None if pycurl else False
 
 def try_use_http2(curl):
@@ -66,7 +65,7 @@ class ResponseManager:
     self.req = req
 
   async def __aenter__(self):
-    return await client.fetch(self.req)
+    return await AsyncHTTPClient().fetch(self.req)
 
   async def __aexit__(self, exc_type, exc, tb):
     pass

tests/conftest.py

@@ -54,12 +54,10 @@ async def get_version():
 
   return __call__
 
-@pytest.fixture(scope="session")
+@pytest.fixture(scope="module")
 def event_loop(request):
   """Override pytest-asyncio's event_loop fixture,
      Don't create an instance of the default event loop for each test case.
-
-     The scope is session because the Tornado AsyncHTTPClient singleton remains.
   """
   loop = asyncio.get_event_loop()
   yield loop