TT#113650 Upgrade pep8 v1.6.2 to pycodestyle from bullseye

We use quite old pep8 v1.6.2 which in particular misses this
change https://github.com/PyCQA/pycodestyle/pull/502
Let's update Dockerfile to use pycodestyle(former pep8) from
bullseye.

Change-Id: Id29acd80be212cffa127550d9a4deb764a5b5b30

t/tap-tests-docker/Dockerfile

@@ -1,23 +1,11 @@
-# DOCKER_NAME=jenkins-tap-test-jessie
-FROM docker.mgm.sipwise.com/sipwise-jessie:latest
+# DOCKER_NAME=jenkins-tap-test-bullseye
+FROM docker.mgm.sipwise.com/sipwise-bullseye:latest
 
 # Important! Update this no-op ENV variable when this Dockerfile
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2020-01-24
-
-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 && \
-  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 && \
-  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
+ENV REFRESHED_AT 2021-08-02
 
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
   binutils \
@@ -25,7 +13,7 @@ RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
   jenkins-debian-glue \
   libperl-critic-perl \
   parallel \
-  pep8 \
+  pycodestyle \
   ruby \
   shellcheck \
   xz-utils && \
@@ -55,12 +43,12 @@ WORKDIR /code/
 # (jump to the next section if you don't want to build yourself!):
 # NOTE: run the following command from root folder of git repository:
 #
-# % docker build --tag="jenkins-tap-test-jessie" -f t/tap-tests-docker/Dockerfile .
-# % docker run --rm -i -t -v $(pwd):/code/source:rw jenkins-tap-test-jessie:latest bash
+# % docker build --tag="jenkins-tap-test-bullseye" -f t/tap-tests-docker/Dockerfile .
+# % docker run --rm -i -t -v $(pwd):/code/source:rw jenkins-tap-test-bullseye:latest bash
 #
 # Use the existing docker image:
-# % docker pull docker.mgm.sipwise.com/jenkins-tap-test-jessie
-# % docker run --rm -i -t -v $(pwd):/code/source:rw docker.mgm.sipwise.com/jenkins-tap-test-jessie:latest bash
+# % docker pull docker.mgm.sipwise.com/jenkins-tap-test-bullseye
+# % docker run --rm -i -t -v $(pwd):/code/source:rw docker.mgm.sipwise.com/jenkins-tap-test-bullseye:latest bash
 #
 # Inside docker (the command is in history, just press UP button):
 #   LC_ALL=C.UTF-8 SHELL=bash WORKSPACE=/code/ /usr/bin/tap_tool_dispatcher