docs: fix legacy Python README link

[JSap0914]

Fixes #50.

What changed:

• Updated the README legacy Python note to link to the v2.5.3 source tree, the last tag verified to contain input_viewer_python.

Why:

• The current main branch no longer contains input_viewer_python, so the existing relative link resolves to 404.

Verification:

• curl -I -L https://github.com/Lab271/input-viewer/tree/main/input_viewer_python returned HTTP 404 before the change.
• git ls-tree --name-only v2.5.3 input_viewer_python shows the directory exists in v2.5.3.
• curl -I -L https://github.com/Lab271/input-viewer/tree/v2.5.3/input_viewer_python returned HTTP 200.
• git diff --check HEAD~1 HEAD.

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