fix(ci): install flake8 and pytest dependencies in workflow

Author: andrepsousa

.github/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r requirements.txt
+          pip install flake8 pytest pytest-cov
 
       - name: Lint (flake8)
         run: flake8 src tests