Fix hardcoded history manager

[yetanotherape]

Related Issue

#540

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 #542 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   97.71%   97.72%   +0.01%     
==========================================
  Files          17       17              
  Lines         830      834       +4     
  Branches      114      114              
==========================================
+ Hits          811      815       +4     
  Misses          8        8              
  Partials       11       11

Impacted Files	Coverage Δ
simple_history/admin.py	98.07% <100%> (+0.03%)	⬆️
...ory/management/commands/clean_duplicate_history.py	100% <100%> (ø)	⬆️
simple_history/utils.py	100% <100%> (ø)	⬆️

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[rossmechanic]

Great! Thanks for this