adding CI script with tests

.gitlab-ci.yml

+stages:
+    - tests
+
+tests:
+    image: git.jinr.ru:5005/gna/gna-base-docker-image:latest
+    stage: tests
+
+    script:
+    - python3 -m pip install -r dagflow/requirements.txt
+    - coverage run --source=. -m pytest
+    - coverage report
+    - coverage xml
+    coverage: '/(?i)total.*? (100(?:\.0+)?\%|[1-9]?\d(?:\.\d+)?\%)$/'
+    artifacts:
+        paths:
+            - tests
+        reports:
+            coverage_report:
+                coverage_format: cobertura
+                path: coverage.xml
+    only:
+        - master
+        - merge_requests

pytest.ini

+[pytest]
+testpaths=tests
+; uncomment below to include coverage into default pytest run
+; addopts= --cov-report term --cov=./ --cov-report xml:cov.xml