Fix RadioButtons double event emissions

[tlambert03]

fixes #188

This introduces a new problem, which is that if someone programmatically sets widget.value to None, then no event will be emitted (in the GUI, this isn't possible since once a radio button is checked it can't be unchecked). quick solutions to this are made slightly challenging by the fact that we don't know if a non-None value is about to be set in the near future. Anyway, that's probably less problematic than the current behavior, so we'll leave that fix for a later PR.

[codecov]

Codecov Report

Merging #189 (965cbf4) into master (260f63e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   90.07%   90.07%   -0.01%     
==========================================
  Files          29       29              
  Lines        2982     2981       -1     
==========================================
- Hits         2686     2685       -1     
  Misses        296      296              

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.87% <100.00%> (-0.03%)	⬇️

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