Ok back to business, add the python-tk package required by matplotlib (six)

.gitlab-ci.yml

 myplotlib_server_client:
   script:
     - apt-get update -q -y
-    - apt-get install python-dev -q -y
+    - apt-get install python-dev python-tk -q -y
     - wget https://bootstrap.pypa.io/get-pip.py
     - python get-pip.py
     - python -m pip --version