[ci] add a simple CI pipeline to run the tests

.gitlab-ci.yml

+stages:
+    - "test"
+
+
+.test: &test
+    before_script:
+        - "pip install --upgrade pip"
+        - "pip install -r requirements.txt"
+        - "pip install .[tests]"
+        - "pip install pytest pytest-cov"
+    script:
+        - >
+            pytest -v --color=yes --cov --cov-report=term
+            --cov-report=xml --junit-xml=pytest-xunit.xml
+    artifacts:
+        reports:
+            junit: pytest-xunit.xml
+            cobertura: coverage.xml
+
+
+python3.7:
+    stage: "test"
+    image: "python:3.7"
+    <<: *test