mypy: disallow untyped calls

report an error whenever a function with type annotations calls a
function defined without annotations.

Also include exceptions.py in mypy checks.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

setup.cfg

@@ -14,7 +14,8 @@ warn_unused_ignores = True
 warn_unreachable = True
 strict_equality = True
 disallow_untyped_defs = True
-files = tuf/api/
+disallow_untyped_calls = True
+files = tuf/api/, tuf/exceptions.py
 
 [mypy-securesystemslib.*]
 ignore_missing_imports = True