feat: speed up unpack_variants

[bdraco]

This function needs to only unpack very specific types since the data is coming from the unmarshaller. The isinstance checks are not needed here and a type check is all that is needed. This speeds up the run by 30%

[codecov]

Codecov Report

Base: 81.42% // Head: 81.52% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (ca1b1d3) compared to base (9fa697d).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   81.42%   81.52%   +0.10%     
==========================================
  Files          26       26              
  Lines        2994     2994              
  Branches      578      578              
==========================================
+ Hits         2438     2441       +3     
+ Misses        343      340       -3     
  Partials      213      213              

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/unpack.py	100.00% <100.00%> (ø)
src/dbus_fast/message_bus.py	71.00% <0.00%> (-0.38%)	⬇️
src/dbus_fast/glib/message_bus.py	83.19% <0.00%> (+2.04%)	⬆️

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