Upgrade Mockito to version 2.7.21

[oscerd]

Just an upgrade. Maybe it's time for a new release?

[codecov-io]

Codecov Report

Merging #306 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #306   +/-   ##
=========================================
  Coverage     78.19%   78.19%           
  Complexity      133      133           
=========================================
  Files            11       11           
  Lines           711      711           
  Branches         77       77           
=========================================
  Hits            556      556           
  Misses          112      112           
  Partials         43       43

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