CI: Doesn't make a difference, but using py36 over 35 for CI tasks

.gitlab-ci.yml

@@ -191,7 +191,7 @@ build-doc:
    - docker
 
   stage: doc
-  image: python:3.5
+  image: python:3.6
 
   script:
     - bash ./.ci/build_doc.sh
@@ -212,7 +212,7 @@ build-pypi-dist:
   <<: *patch_version
 
   stage: build
-  image: python:3.5
+  image: python:3.6
 
   tags:
     - docker
@@ -236,7 +236,7 @@ deploy-doc:
   <<: *setup_ssh
   stage: deploy
   when:  manual
-  image: python:3.5
+  image: python:3.6
 
   tags:
     - docker
@@ -253,7 +253,7 @@ deploy-pypi:
   <<: *setup_ssh
   stage: deploy
   when:  manual
-  image: python:3.5
+  image: python:3.6
 
   tags:
     - docker