Github workflow to use pytest

pytest discovers all unit tests (both unittest and pytest).
unittest only discovered its own.

.github/workflows/push.yml

@@ -23,13 +23,14 @@ jobs:
         pip install -e .
         pip install -r requirements.opt.txt
         pip install flake8==4.0.1
+        pip install pytest==7.0.1 pytest-flake8==1.1.1
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8
       run: |
         flake8 --max-line-length 120 .
     - name: Unit tests
       run: |
-        python -m unittest discover
+        python -m pytest
 # ## Broken in FoTraNMT
 #    - name: Test vocabulary build
 #      run: |
@@ -410,4 +411,4 @@ jobs:
         GITHUB_TOKEN:  ${{ secrets.GITHUB_TOKEN }}
         BRANCH: gh-pages
         FOLDER: docs/build/html
-        CLEAN: true
+        CLEAN: true