chore: separate runtime and dev dependencies, add pytest config and CI cache

README.md

@@ -50,11 +50,13 @@ docker run --rm -p 110:110 -p 25:25 \
 
 ## Tests
 
-Run tests locally with:
+Install development dependencies and run the test suite:
 
 ```bash
+python -m venv .venv
+source .venv/bin/activate
 python -m pip install --upgrade pip
-pip install -r requirements.txt
+pip install -r requirements-dev.txt
 pytest -q
 ```