Changes from comments on PR ethereum#3180

tox.ini

@@ -21,7 +21,7 @@ install_command=python -m pip install {opts} {packages}
 usedevelop=True
 commands=
     core: pytest {posargs:tests/core -m "not asyncio" -k "not async"}
-    core_async: pytest {posargs:tests/core -k "asyncio or async"}
+    core_async: pytest {posargs:tests/core -m asyncio -k async}
     ens: pytest {posargs:tests/ens --ignore=tests/ens/normalization/test_normalize_name_ensip15.py}
     ensip15: pytest {posargs:tests/ens/normalization/test_normalize_name_ensip15.py -q}
     ethpm: pytest {posargs:tests/ethpm}