tox: list Python 3.7 environment (core module only).

Author: veox

tox.ini

@@ -1,5 +1,6 @@
 [tox]
 envlist=
+    py37-core
     py{35,36}-{core,database,transactions,vm,native-blockchain}
     py{36}-{benchmark,p2p,trinity}
     py{36}-rpc-blockchain
@@ -45,6 +46,7 @@ deps = .[eth-extras, test]
 basepython =
     py35: python3.5
     py36: python3.6
+    py37: python3.7
 
 
 [testenv:py36-docs]