Missing mypy stubs in for dateutil.relativedelta

Mypy reported a couple of errors for missing stubs on GitHub
https://github.com/theupdateframework/python-tuf/runs/4431787287?check_suite_focus=true

The error noted that:
"tests/test_api.py:19: error: Library stubs not installed for
"dateutil.relativedelta" (or incompatible with Python 3.10)"
In order to resolve it I added this library to the list of libraries
that we ignore their missing stubs or types.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

pyproject.toml

@@ -88,6 +88,7 @@ exclude=".*_old.py"
 [[tool.mypy.overrides]]
 module = [
   "securesystemslib.*",
-  "urllib3.*"
+  "urllib3.*",
+  "dateutil.relativedelta",
 ]
 ignore_missing_imports = "True"