feat: speed up creating Variant objects

[bdraco]

This is only a 1.5% speed up but we create a lot of these

[codecov]

Codecov Report

Base: 81.85% // Head: 81.93% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (3d87051) compared to base (f438c36).
Patch coverage: 90.00% of modified lines in pull request are covered.

❗ Current head 3d87051 differs from pull request most recent head 89bcef1. Consider uploading reports for the commit 89bcef1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   81.85%   81.93%   +0.07%     
==========================================
  Files          27       27              
  Lines        3109     3105       -4     
  Branches      643      642       -1     
==========================================
- Hits         2545     2544       -1     
+ Misses        347      344       -3     
  Partials      217      217              

Impacted Files	Coverage Δ
src/dbus_fast/signature.py	62.28% <90.00%> (+3.32%)	⬆️
src/dbus_fast/glib/message_bus.py	80.32% <0.00%> (-2.05%)	⬇️
src/dbus_fast/_private/unmarshaller.py	95.29% <0.00%> (-0.05%)	⬇️

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[bdraco]

Tested in production 👍