Fix/use qtpy instead of PyQt5

[schoinh]

Closes #129

Because we were using PyQt5 directly instead of using qtpy (abstraction layer), sometimes the plugin wasn't able to open depending on the user's environment. More info: https://napari.org/plugins/stable/best_practices.html#don-t-include-pyside2-or-pyqt5-in-your-plugin-s-dependencies

Changes:

• Replace all imports of PyQt5 with qtpy
• Import Signal instead of qtpySignal from QtCore (necessary after the above change)
• Use exec_() instead of exec() for QDialog widgets (per napari's advice, and it matches the Qt docs. Not positive this affects the user's ability to open the plugin, but seems like a good thing to do)

Pull request recommendations:

• [x] Name your pull request your-development-type/short-description. Ex: feature/read-tiff-files
• [x] Link to any relevant issue in the PR description. Ex: Resolves [gh-12], adds tiff file format support
• [x] Provide context of changes.
• [ ] Provide relevant tests for your feature or bug fix.
• [ ] Provide or update documentation for any feature added by your pull request.

Thanks for contributing!

[codecov[bot]]

Codecov Report

Merging #130 (2d850ec) into main (1411ce2) will increase coverage by 0.03%. The diff coverage is 93.54%.

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   84.66%   84.70%   +0.03%     
==========================================
  Files          34       34              
  Lines        1435     1432       -3     
==========================================
- Hits         1215     1213       -2     
+ Misses        220      219       -1     

Impacted Files	Coverage Δ
...egmenter/controller/batch_processing_controller.py	65.06% <33.33%> (-0.38%)	:arrow_down:
napari_allencell_segmenter/_dock_widget.py	45.45% <100.00%> (ø)
napari_allencell_segmenter/core/view.py	89.28% <100.00%> (ø)
napari_allencell_segmenter/util/ui_utils.py	100.00% <100.00%> (ø)
napari_allencell_segmenter/view/_main_template.py	100.00% <100.00%> (ø)
..._allencell_segmenter/view/batch_processing_view.py	98.75% <100.00%> (+2.27%)	:arrow_up:
...i_allencell_segmenter/view/workflow_select_view.py	96.87% <100.00%> (ø)
...ri_allencell_segmenter/view/workflow_steps_view.py	91.61% <100.00%> (ø)
...lencell_segmenter/widgets/batch_complete_dialog.py	23.33% <100.00%> (ø)
napari_allencell_segmenter/widgets/file_input.py	77.50% <100.00%> (ø)
... and 5 more

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