ci: generate coverage

We don't do anything with it yet, aside from displaying
the report

.github/workflows/main.yml

@@ -12,8 +12,8 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install pathlib mock pytest pyfakefs
+        pip install pathlib mock pytest pytest-cov pyfakefs
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: pytest
       run: |
-        pytest tests
+        pytest --cov --cov-report=xml --cov-report=term tests