remove usage of old mock

[a-detiste]

Hi,

This project actualy already uses unittest.mock in some places:

django-dbbackup/dbbackup/tests/commands/test_dbbackup.py

Line 6 in 87952dc

from unittest.mock import patch

django-dbbackup/dbbackup/tests/commands/test_dbrestore.py

Line 7 in 87952dc

from unittest.mock import patch

https://github.com/testing-cabal/mock

mock is now part of the Python standard library, available as [unittest.mock](https://docs.python.org/dev/library/unittest.mock.html) in Python 3.3 onwards.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.40%. Comparing base (87952dc) to head (99bedc2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   91.40%   91.40%           
=======================================
  Files          19       19           
  Lines         873      873           
  Branches      157      157           
=======================================
  Hits          798      798           
  Misses         40       40           
  Partials       35       35           

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