Add typing to events.py

[tlambert03]

brings over the type hints recently added to events.py in napari

[codecov]

Codecov Report

Merging #181 (83fa523) into master (dfd4e1b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files          29       29           
  Lines        2908     2908           
=======================================
  Hits         2619     2619           
  Misses        289      289           

Impacted Files	Coverage Δ
magicgui/widgets/_protocols.py	99.45% <100.00%> (ø)

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