Fix custom_repr in mock.call objects

[zmievsa]

Without this fix, insert_assert cannot handle mock.call objects due to their weird structure.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (ec406ff) to head (72a7cbe).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   96.29%   96.29%           
=======================================
  Files           8        8           
  Lines         729      729           
  Branches      111      111           
=======================================
  Hits          702      702           
  Misses         21       21           
  Partials        6        6           

Files with missing lines	Coverage Δ
devtools/pytest_plugin.py	88.13% <100.00%> (ø)

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