fix: allow future annotations in ipywidgets backend

[tlambert03]

Because the ipywidgets backend uses type hints to determine the composed widget type:

class _IPyWidget(protocols.WidgetProtocol):
    _ipywidget: ipywdg.Widget

I access the hints at runtime. This makes that more robust and allows __future__ annotations

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (48c94e2) 87.75% compared to head (cce3cef) 87.75%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #609   +/-   ##
=======================================
  Coverage   87.75%   87.75%           
=======================================
  Files          40       40           
  Lines        4705     4705           
=======================================
  Hits         4129     4129           
  Misses        576      576           

Files	Coverage Δ
src/magicgui/backends/_ipynb/widgets.py	61.34% <100.00%> (ø)

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