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 "types-python-dateutil" into
requirements-test.txt

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

requirements-test.txt

@@ -14,3 +14,4 @@ pylint
 mypy
 bandit
 types-requests
+types-python-dateutil