fix py37 tox coverage config

Author: alimanfoo

tox.ini

@@ -10,7 +10,7 @@ envlist = py27, py35, py36, py37, docs
 setenv =
     PYTHONHASHSEED = 42
     # hooks for coverage exclusions based on Python major version
-    py35,py36: PY_MAJOR_VERSION = py3
+    py35,py36,py37: PY_MAJOR_VERSION = py3
     py27: PY_MAJOR_VERSION = py2
 commands =
     python setup.py build_ext --inplace