Coerce RadioButton with Enum/choices to RadioButtons

[tlambert03]

With the potential confusion between widgets.RadioButton (singular) and widget.RadioButtons (plural), this PR coerces a requested widget_type of RadioButton to RadioButtons if it is associated with a choices argument or an Enum dtype.

[codecov]

Codecov Report

Merging #202 (e051d28) into master (f0d25cd) will decrease coverage by 0.05%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
- Coverage   90.10%   90.04%   -0.06%     
==========================================
  Files          29       29              
  Lines        2991     2994       +3     
==========================================
+ Hits         2695     2696       +1     
- Misses        296      298       +2     

Impacted Files	Coverage Δ
magicgui/type_map.py	90.18% <33.33%> (-1.07%)	⬇️

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