mod (ansible#807)

Add `mod` as permitted RST role

This adds mod to the user list of sphinx roles in the flake8 file.  It was being reported by flake8-rst-docstring whcih is being vetted as it is included in the wemake style guide.
This was the error begin reported:
tests/unit/test_utils.py:1:1: RST304 Unknown interpreted text role "mod".
"""Test the functions exposed in the :mod:`~ansible_navigator.utils` subpackage."""
^

Some good info about this is here: peterjc/flake8-rst-docstrings#16

Reviewed-by: Rick Elrod <[email protected]>
Reviewed-by: None <None>

.flake8

@@ -319,6 +319,7 @@ rst-roles =
   data,
   exc,
   meth,
+  mod,
   term,
   py:class,
   py:data,