Fix DeprecationWarning / invalid escape sequence on Python 3.7.x

[jerrcs]

Fixes this error whenever running Django commands:

/usr/local/lib/python3.7/site-packages/auditlog/models.py:358: DeprecationWarning: invalid escape sequence \.
  add_introspection_rules([], ["^auditlog\.models\.AuditlogHistoryField"])

[codecov-io]

Codecov Report

Merging #197 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #197   +/-   ##
=======================================
  Coverage   83.43%   83.43%           
=======================================
  Files          19       19           
  Lines         513      513           
=======================================
  Hits          428      428           
  Misses         85       85

Impacted Files	Coverage Δ
src/auditlog/models.py	83.42% <0%> (ø)	⬆️

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[Linkid]

Hi! Thank you for this PR.
Could you please rebase ?
Thanks

[Linkid]

A rebase is needed.

[Linkid]

Check #342 for the rebase.