feat: add additional typing

[bdraco]

One more step closer to turning on mypy in the CI

[codecov]

Codecov Report

Base: 81.51% // Head: 81.51% // No change to project coverage 👍

Coverage data is based on head (7760cfe) compared to base (b6d4069).
Patch coverage: 85.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   81.51%   81.51%           
=======================================
  Files          25       25           
  Lines        2937     2937           
  Branches      566      566           
=======================================
  Hits         2394     2394           
  Misses        334      334           
  Partials      209      209           

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/signature.py	57.20% <84.37%> (ø)
src/dbus_fast/constants.py	100.00% <100.00%> (ø)
src/dbus_fast/validators.py	83.33% <100.00%> (ø)

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