test: skip aiohttp version that breaks aioresponses (#717)

diff --git a/noxfile.py b/noxfile.py
index d1f61b9..168c1e2 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -32,7 +32,12 @@
     "grpcio",
 ]
 
-ASYNC_DEPENDENCIES = ["pytest-asyncio", "aioresponses", "asynctest"]
+ASYNC_DEPENDENCIES = [
+    "pytest-asyncio",
+    "aioresponses",
+    "asynctest",
+    "aiohttp!=3.7.4.post0",
+]
 
 BLACK_VERSION = "black==19.3b0"
 BLACK_PATHS = [