ci: check types with mypy

This will result in _partial_ type checking of the type annotations by
using mypy. Keep in mind though that mypy is performing _static_
analysis in a _dynamic_ language so it can only partially check for
correctness.

Some other tool(s) will be needed to have more complete type checking at
runtime. E.g. [typeguard].

[typeguard]: https://pypi.org/project/typeguard/

Author: muggenhor

.github/workflows/pythonpackage.yml

@@ -47,6 +47,11 @@ jobs:
         pip install flake8
         # stop the build if there are Python syntax errors or undefined names
         flake8 --ignore=W293,E265,E266,W503,W504,E731 --count  --show-source --statistics
+    - name: Check types with mypy
+      run: |
+        set -x
+        pip install tox
+        tox -e type
     - name: Test with nose
       run: |
         set -x