fix: Also drop aiohttp from the test matrix for 3.8

Author: untitaker

tox.ini

@@ -33,7 +33,7 @@ envlist =
     {pypy,py2.7,py3.5,py3.6}-rq-{0.6,0.7,0.8,0.9,0.10,0.11}
     {pypy,py2.7,py3.5,py3.6,py3.7,py3.8}-rq-0.12
 
-    {py3.7,py3.8}-aiohttp
+    py3.7-aiohttp
     {py3.7,py3.8}-tornado
 
 [testenv]