Add pylint to the virtualenv.

requirements.txt

 alabaster==0.7.7
+astroid==1.4.5
 Babel==2.3.3
+colorama==0.3.7
 docutils==0.12
 imagesize==0.7.0
 Jinja2==2.8
+lazy-object-proxy==1.2.2
 MarkupSafe==0.23
 py==1.4.31
 Pygments==2.1.3
+pylint==1.5.5
 pytest==2.9.1
 pytz==2016.3
 PyXB==1.2.4
@@ -13,4 +17,5 @@ six==1.10.0
 snowballstemmer==1.2.1
 Sphinx==1.4.1
 sphinx-rtd-theme==0.1.9
+wrapt==1.10.8
 -e .