test: change pytest testing plugin

[tlambert03]

changing to pytest-mypy-testing, I think i prefer the setup

[codecov]

Codecov Report

Merging #438 (0f444fd) into main (919acb2) will decrease coverage by 0.00%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
- Coverage   88.99%   88.99%   -0.01%     
==========================================
  Files          30       30              
  Lines        3935     3944       +9     
==========================================
+ Hits         3502     3510       +8     
- Misses        433      434       +1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.95% <75.00%> (-0.06%)	⬇️
magicgui/backends/_qtpy/application.py	100.00% <100.00%> (ø)
magicgui/widgets/_bases/widget.py	87.91% <100.00%> (+0.20%)	⬆️
magicgui/widgets/_protocols.py	100.00% <100.00%> (ø)

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