Install twisted for narativedocs.

tox.ini

@@ -47,7 +47,7 @@ extras =
 
     {alldeps,tests}-macos: osx_platform
 
-    ; Documentation
+    ; Documentation needs Twisted install to get the version.
     narrativedocs: dev
 
 ;; dependencies that are not specified as extras
@@ -150,6 +150,9 @@ skip_install = true
 [testenv:apidocs]
 basepython=python2.7
 
+[testenv:newsfragment]
+basepython=python2.7
+
 [testenv:manifest-checker]
 basepython=python2.7
 skip_install = true
@@ -162,10 +165,6 @@ skip_install = true
 basepython=python2.7
 skip_install = true
 
-# Sphinx documentation also don't need the code installed.
-[testenv:narrativedocs]
-basepython=python2.7
-skip_install = true
 
 # TwistedChecker needs to be 3.5
 [testenv:txchecker-travis-required]