Add ngclient to mypy

Extend mypy to icnlude all files under ngclient.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

setup.cfg

@@ -15,7 +15,10 @@ warn_unreachable = True
 strict_equality = True
 disallow_untyped_defs = True
 disallow_untyped_calls = True
-files = tuf/api/, tuf/exceptions.py
+files =
+  tuf/api/,
+  tuf/ngclient,
+  tuf/exceptions.py
 
 [mypy-securesystemslib.*]
 ignore_missing_imports = True