Using python 3.5 image to build the doc

The image also have make installed, so we just need to install sphinx. Sphinx image also exists, but less official so I prefered using python:3.5 as in https://gitlab.pasteur.fr/hub/12765-viralhostrangedb/blob/master/.gitlab-ci.yml#L133-145

.gitlab-ci.yml

@@ -42,21 +42,10 @@ test-python37:
   - python setup.py test
 
 pages:
-  image: centos:centos7
+  image: python:3.5
   stage: deploy
   script:
-  - yum install -y epel-release wget
-  - wget https://centos7.iuscommunity.org/ius-release.rpm
-  - rpm -Uvh ius-release*.rpm
-  - yum provides python3.5
-  - yum install -y python35u python35u-libs python35u-devel python35u-pip
-  - pip3.5 install -U pip
-  - pip3.5 install virtualenv
-  - virtualenv -p python3.5 venv
-  - . venv/bin/activate
-  - pip3.5 install -r requirements.txt
-  - pip3.5 install sphinx sphinx-argparse sphinxcontrib.bibtex sphinx_rtd_theme
-  - yum install -y make
+  - pip install sphinx sphinx-argparse sphinxcontrib.bibtex sphinx_rtd_theme
   - cd doc
   - make html
   - mv build/html/ ../public