set pluggy version to match travis-tox requirement

SharpBit · SharpBit · commit e84917872cbe · 2019-09-16T21:00:35.000-04:00
diff --git a/tests/requirements.txt b/tests/requirements.txt
@@ -2,4 +2,5 @@ python-dotenv
 flake8
 asynctest
 pytest
-tox-travis
+tox-travis
+pluggy>=0.12.0,<1.0.0
