Fix pk error

[ddusi]

Description

  File "/Users/ddusi/projects/tboard-backend/app/utils/django/models.py", line 226, in post_create_historical_record_callback
    update_change_reason(instance, str(field_diff))
  File "/Users/ddusi/.pyenv/versions/tboard-backend/lib/python3.7/site-packages/simple_history/utils.py", line 14, in update_change_reason
    manager = instance if instance.id is not None else model
AttributeError: 'RegionManagement' object has no attribute 'id'

i found a bug.
I modified the bug to pk that finds the primary key only with id when customizing it.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

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]

Codecov Report

Merging #806 (5b58deb) into master (3b97e7e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #806   +/-   ##
=======================================
  Coverage   97.68%   97.68%           
=======================================
  Files          19       19           
  Lines         994      994           
  Branches      151      151           
=======================================
  Hits          971      971           
  Misses         10       10           
  Partials       13       13           

Impacted Files	Coverage Δ
simple_history/utils.py	100.00% <100.00%> (ø)

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