feat: add more typing to unmarshaller

[bdraco]

No description provided.

[codecov]

Codecov Report

Base: 81.68% // Head: 81.70% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (e427195) compared to base (a6a248b).
Patch coverage: 95.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   81.68%   81.70%   +0.01%     
==========================================
  Files          25       25              
  Lines        2971     2973       +2     
  Branches      574      574              
==========================================
+ Hits         2427     2429       +2     
  Misses        335      335              
  Partials      209      209              

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/_private/unmarshaller.py	96.52% <100.00%> (ø)
src/dbus_fast/aio/message_bus.py	86.78% <100.00%> (+0.05%)	⬆️
src/dbus_fast/glib/message_bus.py	83.19% <100.00%> (+0.06%)	⬆️

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