Use tox's new env support

http://tox.readthedocs.io/en/latest/config.html#confval-extras=MULTI-LINE-LIST

tox.ini

@@ -38,15 +38,21 @@ envlist=
 
 
 [testenv]
+;; dependencies managed by extras in t.p._setup.py._EXTRAS_REQUIRE
+extras =
+    {alldeps,tests,coverage}: all_non_platform
+
+    windows: windows_platform
+
+    {alldeps,tests}-macos: osx_platform
+
+    ; Documentation
+    narrativedocs: dev
+
+;; dependencies that are not specified as extras
 deps =
-    {alldeps,tests,coverage}: .[all_non_platform]
-
     py27-{alldeps,tests,coverage}-{posix,macos}: pysqlite
 
-    windows: .[windows_platform]
-
-    {alldeps,tests}-macos: .[osx_platform]
-
     {withcov,coverage}: coverage
 
     codecov-publish: codecov
@@ -60,9 +66,6 @@ deps =
     {twistedchecker,txchecker}: twistedchecker>=0.6.0
     txchecker: diff_cover
 
-    ; Documentation
-    narrativedocs: .[dev]
-
 ; All environment variables are passed.
 passenv = *