chore(yamllint): add and integrate yamllint

.gitlab-ci.yml

 image: python
 
 stages:
+  - lint
   - tests
   - after-tests
 
@@ -50,3 +51,14 @@ check-manifest:
 #   script:
 #   # regarding the " everywhere when there is a '%%': yaml is extremly weird when %% are involved
 #   - curl -X POST -d "token=${READTHEDOCS_TOKEN}" https://readthedocs.org/api/v2/webhook/cubicweb_%(cubename)s/${READTHEDOCS_ID} -w "\nhttp code:"\ "%%{http_code}\n" -f
+
+yamllint:
+  stage: lint
+  rules:
+    - changes:
+        - "**/*.yaml"
+        - "*.yaml"
+        - "**/*.yml"
+        - "*.yml"
+  script:
+    - tox -e yamllint

.yamllint

+extends: default
+
+rules:
+  document-start: disable
+  line-length: disable
+  indentation:
+    spaces: 2

MANIFEST.in

@@ -11,3 +11,4 @@ include cubicweb-skos.spec
 prune cubicweb-skos.spec
 prune debian
 exclude .gitlab-ci.yml
+exclude .yamllint

tox.ini

 [tox]
-envlist = py27,py3,flake8,check-manifest
+envlist = py27,py3,flake8,check-manifest,yamllint
 
 [testenv]
 deps =
@@ -62,3 +62,9 @@ commands =
   sh -c "PACKAGE_NAME=$(python3 setup.py --name) && VERSION=$(python3 setup.py --version) &&   cd dist &&   tar xf $PACKAGE_NAME-$VERSION.tar.gz &&   cd $PACKAGE_NAME-$VERSION &&   cp -a {toxinidir}/debian . &&   mk-origtargz --rename ../$PACKAGE_NAME-$VERSION.tar.gz &&   dpkg-buildpackage -us -uc --no-check-builddeps --build=source "
   sh -c "cd dist && dcmd zip latest.zip *.changes"
   http -f POST https://{env:JENKINS_USER}:{env:JENKINS_TOKEN}@jenkins.intra.logilab.fr/job/pkg-from-dsc/buildWithParameters DIST=buster source.zip@dist/latest.zip REPO=buster PUBLISH=true
+
+[testenv:yamllint]
+skip_install = true
+deps = yamllint
+commands =
+  yamllint .