Fix extreme float values for slider and spinbox

[tlambert03]

fixes #177 as well as a bug when using a FloatSlider with float values greater than 2^32 / 2

[codecov]

Codecov Report

Merging #178 (eba3c9e) into master (c4294a3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #178   +/-   ##
=======================================
  Coverage   90.05%   90.06%           
=======================================
  Files          29       29           
  Lines        2855     2908   +53     
=======================================
+ Hits         2571     2619   +48     
- Misses        284      289    +5     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.95% <100.00%> (+0.15%)	⬆️
magicgui/widgets/_bases/ranged_widget.py	85.56% <100.00%> (+0.95%)	⬆️
magicgui/widgets/_concrete.py	84.22% <100.00%> (+2.25%)	⬆️
magicgui/widgets/_transforms.py	59.52% <0.00%> (-14.29%)	⬇️
magicgui/_util.py	60.60% <0.00%> (-1.52%)	⬇️
magicgui/widgets/_function_gui.py	96.87% <0.00%> (+0.62%)	⬆️
magicgui/type_map.py	91.13% <0.00%> (+0.63%)	⬆️

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