Use unittest.mock instead of mock

[WhyNotHugo]

This module has been included in Python itself since 3.3.

Fixes: #1646

[codecov]

Codecov Report

Merging #1673 (42329a1) into master (87bacb6) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1673      +/-   ##
==========================================
+ Coverage   95.40%   95.51%   +0.11%     
==========================================
  Files          47       47              
  Lines        7209     7209              
==========================================
+ Hits         6878     6886       +8     
+ Misses        331      323       -8     

Impacted Files	Coverage Δ
tests/test_decorator.py	89.63% <100.00%> (ø)
tests/test_queue.py	100.00% <100.00%> (ø)
tests/test_scheduler.py	95.39% <100.00%> (ø)
tests/test_sentry.py	97.36% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (ø)
tests/test_worker.py	97.54% <100.00%> (ø)
tests/test_worker_registration.py	100.00% <100.00%> (ø)
rq/worker.py	88.70% <0.00%> (+0.13%)	⬆️
rq/job.py	97.85% <0.00%> (+0.32%)	⬆️
rq/queue.py	93.88% <0.00%> (+1.06%)	⬆️
... and 1 more

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[selwin]

Thanks!