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

Merging #146 (72a7cbe) into main (ec406ff) will not change coverage.
The diff coverage is 100.00%.

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	Coverage Δ
devtools/pytest_plugin.py	88.13% <100.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec406ff...72a7cbe. Read the comment docs.