ci: add basic job for checking types

Author: G-Rath

.github/workflows/python-app.yml

@@ -30,3 +30,21 @@ jobs:
     - name: Build
       run: |
         python -m build -nwsx .
+
+  types:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install --upgrade setuptools setuptools_scm wheel build tox mypy
+    - name: Check types
+      run: |
+        mypy .