fix: conditional Self import depending on python version

[mdaubie]

Description

Fixes #110

Notes

Tested for:

• Python 3.10:

  uv run --no-dev --python 3.10 python -c "import pymkv"

• Python 3.11:

  uv run --no-dev --python 3.11 python -c "import pymkv"

I don't think we can implement actual tests because typing-extension is always present in dev runtimes.
Could use dedicated tools though, like deptry.

[GitBib]

Thanks for the quick fix! Verified the bug reproduces on Python 3.10 with a no-dev install and that this PR resolves it on both 3.10 (typing-extensions installed via marker) and 3.11+ (uses stdlib typing.Self, no extra dep).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.11%. Comparing base (a9d20cf) to head (adc55d1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files          21       21           
  Lines        1472     1475    +3     
=======================================
+ Hits         1459     1462    +3     
  Misses         13       13           

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