add dev requirements

.gitlab-ci.yml

@@ -3,6 +3,7 @@ image: "python:3.8"
 before_script:
   - python --version
   - pip install -r requirements.txt
+  - pip install -r dev-requirements.txt
   
 stages:
   - lint

dev_requirements.txt

@@ -0,0 +1,11 @@
+coverage==5.3
+flake8==3.8.4
+flake8-polyfill==1.0.2
+mypy==0.790
+mypy-extensions==0.4.3
+pycodestyle==2.6.0
+pyflakes==2.2.0
+pylint==2.6.0
+pytest==6.1.2
+pytest-cov==2.10.1
+pytest-datafiles==2.0