refactor: enable strict mypy type checking for most modules

[tlambert03]

tried hard not to touch anything functional. Should be mostly just typing, with a couple of sourcery updates

[codecov]

Codecov Report

Base: 89.78% // Head: 89.83% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (24c80c8) compared to base (f46c574).
Patch coverage: 95.25% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   89.78%   89.83%   +0.04%     
==========================================
  Files          37       37              
  Lines        4456     4407      -49     
==========================================
- Hits         4001     3959      -42     
+ Misses        455      448       -7     

Impacted Files	Coverage Δ
src/magicgui/_util.py	74.64% <78.57%> (+0.73%)	⬆️
src/magicgui/widgets/_dialogs.py	80.95% <80.00%> (-0.87%)	⬇️
src/magicgui/signature.py	96.62% <85.71%> (ø)
src/magicgui/type_map/_type_map.py	96.48% <88.88%> (ø)
src/magicgui/widgets/bases/_ranged_widget.py	88.70% <89.47%> (+0.47%)	⬆️
src/magicgui/widgets/bases/_categorical_widget.py	84.50% <91.66%> (+0.50%)	⬆️
src/magicgui/widgets/_table.py	96.14% <91.80%> (-0.05%)	⬇️
src/magicgui/widgets/_function_gui.py	93.69% <93.33%> (-0.06%)	⬇️
src/magicgui/widgets/bases/_widget.py	87.93% <95.83%> (ø)
src/magicgui/widgets/_concrete.py	88.93% <96.49%> (-0.18%)	⬇️
... and 28 more

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