setup ci for linting

.gitlab-ci.yml

+stages:
+  - test
+
+default:
+  tags:
+    - ci.inria.fr
+    - linux
+    - small
+
+lint:
+  stage: test
+  image: pyfound/black:23.9.1
+  script:
+    - black --check src/aevol/
+
+pytest:
+  stage: test
+  image: python
+  before_script:
+    - pip install pytest
+  script:
+    - pip install .
+    - cd tests
+    - pytest