ci: put back pytest

.github/workflows/tests.yml

@@ -33,10 +33,21 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
-    - name: Run tox in this environment
+    - name: Installation (deps and package)
+      # we install with flit --pth-file,
+      # so that coverage will be recorded for the module
+      run: |
+        pip install flit
+        flit install --deps=production --extras=test --pth-file
+
+    - name: Run pytest
+      run: |
+        pytest --cov=mdformat_pelican --cov-report=xml --cov-report=term-missing
+
+    - name: Run pytest with mdformat-gfm
       run: |
-        python -m pip install tox
-        tox
+        pip install mdformat-gfm
+        pytest --cov=mdformat_pelican --cov-report=xml --cov-report=term-missing
 
   pre-commit-hook:
     runs-on: ubuntu-latest