Run stubtest for reclass_rs and mypy for the Python tests in CI

We extend the x86_64 Python workflow to run `stubtest` and `mypy` to get
basic validation for the type stubs. This should be sufficient to catch
PRs that introduce new Python API surface without adding type stubs.

Unfortunately, from some limited testing, `stubtest` doesn't necessarily
catch incorrect type stubs, since some of the PyO3 generated functions
apparently have very limited metadata (especially for return types).

Author: simu

.github/workflows/python.yml

@@ -94,7 +94,9 @@ jobs:
           python3 -m venv .venv
           source .venv/bin/activate
           pip install reclass-rs --find-links dist --force-reinstall
-          pip install pytest kapicorp-reclass
+          pip install pytest mypy kapicorp-reclass
+          stubtest reclass_rs
+          mypy --check-untyped-defs --ignore-missing-imports tests/
           pytest
       - name: pytest
         if: ${{ !startsWith(matrix.platform.target, 'x86') && matrix.platform.target != 'ppc64' }}