style: Import MagicMock from unittest.mock

[bdrung]

Importing MagicMock from unittest.mock makes the code shorter by reducing the need to wrap long lines.

[codecov]

Codecov Report

Merging #243 (617f795) into main (c143c7c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   83.73%   83.75%   +0.01%     
==========================================
  Files          93       93              
  Lines       18768    18783      +15     
==========================================
+ Hits        15716    15731      +15     
  Misses       3052     3052              

Files	Coverage Δ
tests/helper.py	98.41% <100.00%> (+0.02%)	⬆️
tests/integration/test_crashdb_launchpad.py	13.92% <100.00%> (+0.16%)	⬆️
tests/integration/test_hookutils.py	93.12% <100.00%> (+0.02%)	⬆️
tests/integration/test_report.py	92.40% <100.00%> (+<0.01%)	⬆️
tests/integration/test_signal_crashes.py	90.75% <100.00%> (+0.01%)	⬆️
tests/integration/test_ui.py	97.45% <100.00%> (+<0.01%)	⬆️
tests/system/test_ui_gtk.py	100.00% <100.00%> (ø)
tests/system/test_ui_kde.py	96.64% <100.00%> (+<0.01%)	⬆️
tests/unit/test_fileutils.py	100.00% <100.00%> (ø)
tests/unit/test_hooks_image.py	100.00% <100.00%> (ø)
... and 5 more