fix: change action type hint on MDAEvent

[tlambert03]

cc @fdrgsp,
I realized we do actually need to use Union types on MDAEvent.action.

this fixes it

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (e50aa35) 96.35% compared to head (61ddd5b) 96.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   96.35%   96.36%   +0.01%     
==========================================
  Files          12       12              
  Lines         714      716       +2     
==========================================
+ Hits          688      690       +2     
  Misses         26       26              

Files Changed	Coverage Δ
src/useq/_actions.py	100.00% <100.00%> (ø)
src/useq/_mda_event.py	86.44% <100.00%> (ø)

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