[PR #5940/2b8ac3c6 backport][stable-6] Remove unneccessary imports (#5976)

Remove unneccessary imports (#5940)

* Remove unneccessary imports.

* Keep unnecessary imports in module_utils - for now.

* Make older sanity tests shut up.

* Also make flake8 happier.

(cherry picked from commit 2b8ac3c629)

Co-authored-by: Felix Fontein <felix@fontein.de>

plugins/callback/mail.py

@@ -79,7 +79,6 @@ import re
 import email.utils
 import smtplib
 
-from ansible.module_utils.six import string_types
 from ansible.module_utils.common.text.converters import to_bytes
 from ansible.parsing.ajson import AnsibleJSONEncoder
 from ansible.plugins.callback import CallbackBase