TT#29356 Use pep8 from jessie-backports as requested in TT#23863

Change-Id: I2e98ac581832512a9f498ff53e498813e8ceeff1

t/tap-tests-docker/Dockerfile

@@ -10,6 +10,15 @@ ENV REFRESHED_AT 2018-01-03
 RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
 deb https://deb.sipwise.com/autobuild/ internal-jessie main\n" > /etc/apt/sources.list.d/sipwise-internal.list
 
+RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
+deb [arch=amd64] https://debian.sipwise.com/debian jessie-backports main contrib\n" > /etc/apt/sources.list.d/debian-backports.list
+
+RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
+Explanation: force installing pep8 from jessie-backports\n\
+Package: pep8\n\
+Pin: release n=jessie-backports\n\
+Pin-Priority: 600\n" > /etc/apt/preferences.d/pep8.pref
+
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
   shellcheck pep8 libperl-critic-perl ruby jenkins-debian-glue parallel \
   file binutils xz-utils && \