add python 3.7 to tox

Author: miguelgrinberg

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist=flake8,py27,py34,py35,py36,pypy,pypy3,docs,coverage
+envlist=flake8,py27,py34,py35,py36,py37,pypy,pypy3,docs,coverage
 skip_missing_interpreters=True
 
 [testenv]
@@ -11,22 +11,24 @@ deps=
     coverage
     mock
 basepython =
+    flake8: python3.6
     py27: python2.7
     py34: python3.4
     py35: python3.5
     py36: python3.6
+    py37: python3.7
     pypy: pypy
     pypy3: pypy3
+    docs: python3.6
+    coverage: python3.6
 
 [testenv:flake8]
-basepython=python3.6
 deps=
     flake8
 commands=
     flake8 --exclude=".*" --ignore=E402,E722 socketio tests
 
 [testenv:docs]
-basepython=python2.7
 changedir=docs
 deps=
     sphinx
@@ -36,7 +38,6 @@ commands=
     make html
 
 [testenv:coverage]
-basepython=python3.6
 commands=
     coverage run --branch --source=socketio setup.py test
     coverage html