Merge branch 'update_otbtf_321' into 'develop'

CI: update otbtf version -> 3.2.1

See merge request !43

.gitlab-ci.yml

 default:
-  image: "gitlab-registry.irstea.fr/remi.cresson/otbtf:3.1-cpu-basic-dev"
+  image: "gitlab-registry.irstea.fr/remi.cresson/otbtf/3.2.1:cpu-basic-dev"
   cache:
     key: $CI_COMMIT_REF_SLUG
     paths:
@@ -32,6 +32,7 @@ make_env:
   script:
     - virtualenv --system-site-packages venv
     - source venv/bin/activate
+    - pip install --upgrade pip
     - pip install flake8 pylint codespell pytest pytest-cov
     - wget -P . --no-verbose -e robots=off --recursive --level=inf --no-parent -R "index.html*" -R "LOGO.JPG" --cut-dirs=3 --no-host-directories --content-on-error http://indexof.montpellier.irstea.priv/projets/geocicd/scenes/test_data/