ci: add mypy check in CI

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, "3.10", "3.11"]
+        python-version: [3.7, 3.8, 3.9, "3.10", "3.11", "3.12"]
 
     steps:
       - uses: actions/checkout@v3
@@ -19,11 +19,14 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install flake8
+          python -m pip install --upgrade flake8
+          python -m pip install --upgrade mypy
           python -m pip install poetry
           poetry install -v
       - name: Run flake8
         run: flake8 argx
+      - name: Run mypy
+        run: mypy -p argx
       - name: Test with pytest
         run: poetry run pytest tests/ --junitxml=junit/test-results-${{ matrix.python-version }}.xml
       - name: Upload pytest test results