replace deprecated django.utils.six with six

[erikvw]

Remove deprecated django.utils.six in favor of six

Description

Usage of the vendored library django.utils.six is expected to be removed in Django 3.0.

See Django 3.0 release notes

How Has This Been Tested?

This change does not change any functionality so current tests suffice.

Types of changes

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

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #526 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #526   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files          17       17           
  Lines         830      830           
  Branches      114      114           
=======================================
  Hits          811      811           
  Misses          8        8           
  Partials       11       11

Impacted Files	Coverage Δ
simple_history/models.py	98.95% <100%> (ø)	⬆️

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[rossmechanic]

LGTM. Thanks!