add CI

.gitlab-ci.yml

+stages:
+  - test
+
+test:
+  stage: test
+  image: python:3.6
+  before_script:
+    - pip install pytest-cov
+    - pip install .[test]
+  script:
+    - pytest --cov=./ .
+  only:
+    refs:
+      - main
+      - merge_requests