use unittest.mock instead of mock

[reoono]

What does this PR do?

Changed to use unittest.mock instead of mock.

• Supported versions of Python have unittest as a standard library,
• Unnecessary dependencies can be removed.

Fixes #659

[codecov]

Codecov Report

Merging #668 (59bb483) into main (43b6712) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #668   +/-   ##
=======================================
  Coverage   91.64%   91.64%           
=======================================
  Files          17       17           
  Lines        1568     1568           
=======================================
  Hits         1437     1437           
  Misses        131      131           

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[MSeal]

Thanks! This was a left-over from supporting older Python versions