Remove backported salted_hmac use django provided one instead

[sevdog]

Description

Removes salted_hmac_256 from the codebase, replacing with django.utils.crypto.salted_hmac.

Also a small tweak: using tuple instead of list when there is no need to modify a sequence which is only needed to match an array-like API.

Motivation and Context

As stated in #601: this package now has a minimum requirements for django>=3.2, thus the backported method salted_hmac_256 is not required anymore as we can rely on salted_hmac from django.

How Has This Been Tested?

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue), refactor
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[claudep]

I think a last improvement could be to reference the issue in the commit message (Fixes #601 or something like that) so it will auto-close.

[sevdog]

Done.