fix: result typing in ServiceInterface._handle_signal

[bdraco]

This is another cython3 fix

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.12% 🎉

Comparison is base (c933be7) 82.56% compared to head (442e1a0) 82.68%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
+ Coverage   82.56%   82.68%   +0.12%     
==========================================
  Files          27       27              
  Lines        3212     3212              
  Branches      666      666              
==========================================
+ Hits         2652     2656       +4     
+ Misses        342      341       -1     
+ Partials      218      215       -3     

Files Changed	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/service.py	88.65% <ø> (ø)

... and 1 file with indirect coverage changes

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