added gitlab-ci yaml file

.gitlab-ci.yml

+image: python:3.7-stretch
+
+stages:
+    - style
+    - tests
+
+before_script:
+    - pip install -U pip setuptools
+    - pip install tox
+
+tests:
+    script:
+        - tox -r -vv -e py37
+    stage: tests
+
+style:
+    script:
+        - tox -r -vv -e flake8
+    allow_failure: true
+    stage: style