fix: pin 'aoihttp < 3.7.0dev' (#634)

Working around breaking change in 3.7.0.  See:

https://github.com/pnuckowski/aioresponses/issues/173
diff --git a/noxfile.py b/noxfile.py
index b92f493..3888144 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -30,7 +30,12 @@
     "grpcio",
 ]
 
-ASYNC_DEPENDENCIES = ["pytest-asyncio", "aioresponses", "asynctest"]
+ASYNC_DEPENDENCIES = [
+    "pytest-asyncio",
+    "aiohttp < 3.7.0dev",
+    "aioresponses",
+    "asynctest",
+]
 
 BLACK_VERSION = "black==19.3b0"
 BLACK_PATHS = [