Added Python 3.7 testing

Also made Sphinx 1.8 testing use stable version

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    py{27,34,35,36}-sphinx{13,14,15,16,17,18}
+    py{27,34,35,36,37}-sphinx{13,14,15,16,17,18}
     lint
     docs
 
@@ -10,6 +10,7 @@ python =
     3.4: py34
     3.5: py35
     3.6: py36
+    3.7: py37
 
 [testenv]
 setenv =
@@ -22,13 +23,13 @@ deps = -r{toxinidir}/requirements.txt
     sphinx15: Sphinx<1.6
     sphinx16: Sphinx<1.7
     sphinx17: Sphinx<1.8
-    sphinx18: Sphinx==1.8.0b1
+    sphinx18: Sphinx<1.9
 commands =
     py.test {posargs}
 
 [testenv:docs]
 deps =
-    Sphinx>=1.7,<=1.8
+    Sphinx>=1.8,<=1.9
     sphinx_rtd_theme
 changedir = {toxinidir}/docs
 commands =