Don't install Twisted when not needed.

tox.ini

@@ -131,7 +131,7 @@ commands =
 
     pycodestyle: python {toxinidir}/admin/pycodestyle-twisted.py {posargs:admin src/twisted}
     ; Travis-CI version is executing in diff mode without any input arguments.
-    pycodestylediff: /bin/sh -c "git diff trunk... | python {toxinidir}/admin/pycodestyle-twisted.py --diff"
+    pycodestylediff: /bin/sh -c "git diff trunk | python {toxinidir}/admin/pycodestyle-twisted.py --diff"
 
 
 [testenv:twistedchecker]
@@ -147,6 +147,25 @@ basepython=python2.7
 [testenv:manifest-checker]
 basepython=python2.7
 
+
+# Static checkers don't need to install the source code.
+[testenv:pyflakes3]
+skip_install = true
+[testenv:pyflakes]
+skip_install = true
+[testenv:pycodestyle]
+skip_install = true
+[testenv:pycodestylediff]
+skip_install = true
+[testenv:newsfragment]
+skip_install = true
+
+
+# Sphinx documentation also don't need the code installed.
+[testenv:narrativedocs]
+skip_install = true
+
+
 # TwistedChecker needs to be 3.5
 [testenv:txchecker-travis-required]
 basepython=python3.5