Using Python 3.6 because of OpenCyphal/pydsdl#26

Author: pavel-kirienko

.gitlab-ci.yml

@@ -1,12 +1,13 @@
 before_script:
+  - sudo add-apt-repository ppa:deadsnakes/ppa
   - apt-get update
-  - PATH=$HOME/texlive/bin/x86_64-linux:$PATH
   - apt-get install -y texlive-full
   - apt-get install -y python-pygments
-  - apt-get install -y python3 python3-pip
+  - apt-get install -y python3.6
   - apt-get install -y git
   - apt-get install -y ghostscript
-  - pip3 install -r requirements.txt
+  - update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 10
+  - python3.6 -m pip install -r requirements.txt
 
 job:
   image: ubuntu:16.04