Upgrade ty type checker to latest version and fix mypy configuration

[Copilot]

This PR upgrades the ty type checker to the latest available version and fixes an invalid error code in the mypy configuration that was causing warnings.

Changes Made

Type Checker Upgrade

• Updated ty dependency from >=0.0.1a20 to >=0.0.1a21 (latest version) in pyproject.toml

Mypy Configuration Fix

• Removed invalid deprecated error code from mypy.ini that was causing warnings during type checking

Analysis of Existing ty: ignore Comments

During this upgrade, I thoroughly analyzed all existing ty: ignore comments to determine if they could be removed. After testing with both type checkers, all existing ignore comments are still necessary:

• src/promnesia/cannon.py: The ty: ignore[invalid-return-type] comment addresses a known false positive in ty where it incorrectly thinks urlunsplit() returns bytes instead of str (see ty issue #733)
• src/promnesia/logging.py: The ty: ignore[invalid-assignment] and ty: ignore[missing-argument] comments are legitimately needed for method assignment operations that both type checkers correctly flag as violations

Verification

Both mypy and ty now pass type checking without configuration warnings:

$ mypy --config-file mypy.ini src/promnesia/logging.py src/promnesia/cannon.py
Success: no issues found in 2 source files

$ ty check src/promnesia/logging.py src/promnesia/cannon.py  
All checks passed!

The changes are minimal and maintain backward compatibility while ensuring the project uses the latest type checking tools.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.00%. Comparing base (3ea30a2) to head (be77c8a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #479   +/-   ##
=======================================
  Coverage   83.00%   83.00%           
=======================================
  Files          71       71           
  Lines        5253     5253           
  Branches      145      145           
=======================================
  Hits         4360     4360           
  Misses        748      748           
  Partials      145      145           

Flag	Coverage Δ
mypy-3.10	82.98% <ø> (ø)
mypy-3.11	82.98% <ø> (ø)
mypy-3.12	83.00% <ø> (ø)
mypy-3.13	83.00% <ø> (ø)

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