Fix typesafety checks with numpy 1.20

[tlambert03]

the new numpy release caused a couple new (correct) errors in type hints... which is great! this fixes those so that the typesafety tests can work with numpy 1.20

[codecov]

Codecov Report

Merging #141 (ad7742e) into master (46cabd5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #141   +/-   ##
=======================================
  Coverage   90.97%   90.97%           
=======================================
  Files          25       25           
  Lines        2526     2526           
=======================================
  Hits         2298     2298           
  Misses        228      228           

Impacted Files	Coverage Δ
magicgui/widgets/_table.py	96.80% <100.00%> (ø)

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