Add infrastructure for pytest and mypy

Make mypy run with proper type stubs for micropython RP2.
Add basic structure for pytest testing.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

DEVELOP.md

@@ -0,0 +1,11 @@
+# Developer notes
+
+## How to setup python environment for mypy and pytest
+
+```bash
+cd software
+python -m venv test-venv
+. test-venv/bin/activate
+pip install -r tests/requirements.txt
+pip install -U micropython-rp2-pico_w-stubs --target typings
+```