Drop mock package dependency

[Anthchirp]

and use the standard library implementation (unittest.mock) instead of the mock package.

This is independent of the pytest-mock PRs going around in parallel

Also depends on xia2/xia2#541

[codecov]

Codecov Report

Merging #1500 (b85b57a) into master (38d438f) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1500      +/-   ##
==========================================
- Coverage   65.57%   65.54%   -0.04%     
==========================================
  Files         614      614              
  Lines       68973    68973              
  Branches     9530     9533       +3     
==========================================
- Hits        45232    45207      -25     
- Misses      21902    21928      +26     
+ Partials     1839     1838       -1