Use unittest.mock where possible

[cottsay]

The 'unittest.mock' module has been present since Python 3.3, and some platforms are now dropping the 'mock' backport package.

[codecov-commenter]

Codecov Report

Merging #850 (1f224f6) into master (45290f4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #850   +/-   ##
=======================================
  Coverage   75.00%   75.00%           
=======================================
  Files          42       42           
  Lines        3317     3317           
=======================================
  Hits         2488     2488           
  Misses        829      829           

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[nuclearsandwich]

In the spirit of the stairwell. Does it make sense to drop mock from the CI installation since all our tested platforms provide unittest.mock?

[cottsay]

Does it make sense to drop mock from the CI installation

From an offline discussion: Eventually, but it is still needed for Python 2.7, so it will need to be made conditional for now. I'll make that change in a follow-up PR.