tests/py: Add mypy config for tests

This ensures that our typing information is correct. Unfortunately
we cannot enable mypy checks in pre-commit because it passes the list of
changed python files to it which then ignores our exclude rule.

tests/.mypy.ini

@@ -0,0 +1,15 @@
+[mypy]
+warn_unused_configs = True
+check_untyped_defs=True
+files=.
+exclude = (?x)(
+    templates/.*.py # template files are a bit special
+    |test-document-fuse.py$ # has issues with typing
+  )
+
+[mypy-gi.*]
+ignore_missing_imports = True
+[mypy-dbus.*]
+ignore_missing_imports = True
+[mypy-dbusmock.*]
+ignore_missing_imports = True