feat: add py.typed for typing extensions

Extensions which use the tutor API need to know that the tutor package
is typed. For that we add the py.typed file.

Also, we fix the signature of get_typed.

See:
https://mypy.readthedocs.io/en/stable/installed_packages.html#installed-packages

MANIFEST.in

@@ -1,3 +1,4 @@
 include requirements/base.in
 include requirements/plugins.txt
 recursive-include tutor/templates *
+include tutor/py.typed

tutor/types.py

@@ -35,7 +35,10 @@ def cast_config(config: t.Any) -> Config:
 
 
 def get_typed(
-    config: Config, key: str, expected_type: t.Type[T], default: t.Optional[T] = None
+    config: t.Dict[str, t.Any],
+    key: str,
+    expected_type: t.Type[T],
+    default: t.Optional[T] = None,
 ) -> T:
     value = config.get(key, default)
     if not isinstance(value, expected_type):