christiansandberggh-358: Add mypy CI and config

Estalish a minimal mypy config that ensures annotations are compatible
with Python 3.8 (the oldest supported Python version).

Add a forgiving CI job that does not affect the status of the GitHub
workflow run.

.github/workflows/pythonpackage.yml

@@ -53,3 +53,18 @@ jobs:
       run: python3 -m pip install -r doc/requirements.txt -e .
     - name: Build docs
       run: make -C doc html
+
+  mypy:
+    continue-on-error: true
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-python@v5
+      with:
+        python-version: "3.8"
+        cache: pip
+        cache-dependency-path: |
+          'pyproject.toml'
+          'requirements-dev.txt'
+    - run: pip install -r requirements-dev.txt
+    - run: mypy --config-file pyproject.toml .

pyproject.toml

@@ -48,3 +48,11 @@ testpaths = [
 filterwarnings = [
     "ignore::DeprecationWarning",
 ]
+
+[tool.mypy]
+python_version = "3.8"
+exclude = [
+    "^examples*",
+    "^test*",
+    "^setup.py*",
+]

requirements-dev.txt

@@ -0,0 +1 @@
+mypy~=1.10