Include backports.csv as a python dependency in the wheel.

pkg/pip/setup_pip.py

@@ -79,7 +79,7 @@ setup(
     extras_require={
         # ...
         ":python_version<'2.7'": ["ordereddict"],
-        ":python_version<='2.7'": ["importlib"],
+        ":python_version<='2.7'": ["backports.csv", "importlib"],
     },
 
     # Specify data files to be included. For Python 2.6 include them in MANIFEST.in