Lint requirement file for py2

Read the Docs corporate site still uses Python 2.

Author: humitos

requirements/lint.txt

@@ -1,7 +1,11 @@
 -r pip.txt
+# We need these special cases of Python < 3 for Read the Docs
+# corporate site.
 astroid==2.0.4; python_version > '3'
 astroid==1.6.4; python_version < '3'
-pylint==2.1.1
+pylint==2.1.1; python_version > '3'
+pylint<2; python_version < '3'
+
 pylint-django==2.0.2
 pylint-celery==0.3
 prospector==1.1.2