Add gitlab CI configuration.

.gitlab-ci.yml

 image: clarinpl/python:3.8
 
+cache:
+  paths:
+    - .tox
+
 stages:
   - test
   - deploy
 
 before_script:
+  - pip install tox==2.9.1
   - apt-get update -yqq
     && apt-get install -yqq --no-install-recommends
     make
@@ -15,7 +20,7 @@ before_script:
 unittests:
   stage: test
   script:
-    - python setup.py test
+    - tox -v -e pytest
 
 pypi_deploy:
   before_script: