Enable static type checking

setup.cfg

@@ -5,6 +5,7 @@ test = pytest
 python-tag = py3
 
 [tool:pytest]
+# More options are set in `setup.py` via environment variable `PYTEST_ADDOPTS`
 addopts = --flake8 --isort --cov --cov-report=term --cov-report=xml -r s
 norecursedirs = dist .cache .git build Radicale.egg-info .eggs venv
 
@@ -15,6 +16,10 @@ known_third_party = defusedxml,passlib,pkg_resources,pytest,vobject
 [flake8]
 extend-ignore = H
 
+[mypy]
+ignore_missing_imports = True
+show_error_codes = True
+
 [coverage:run]
 branch = True
 source = radicale