Detect duplicate branch definitions

[JelleZijlstra]

Adds Y069 to flag definitions that are identical across sibling sys.version_info or sys.platform branches. Non-consecutive duplicates in 3+-branch version chains are ignored so newest-first ordering can be preserved.

Some known issues in typeshed are fixed in python/typeshed#15743.

Remaining typeshed hits from this check:

• stdlib/ast.pyi:769 - overlapping ImportFrom.__init__ overload shared by adjacent version branches. Deduplicating this would probably make the code harder to follow.
• stdlib/pathlib/__init__.pyi:160 - Path.__new__ is identical except for a pyright ignore comment, which is ignored by AST comparison.
• stdlib/tkinter/__init__.pyi:791, stdlib/tkinter/__init__.pyi:822 - wm_attributes platform overload sets partially overlap. Hard to fix without making the code harder to follow
• stubs/pyinstaller/PyInstaller/building/api.pyi:33 - _IconParam is the same for win32 and darwin while _Icon differs.

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This found a few useful things, but also has false positives. It may be worth doing to protect against issues in future PRs, but curious to hear other opinions.

[github-actions]

⚠ Flake8 diff showing the effect of this PR on typeshed:

> ./stdlib/ast.pyi:769:9: Y069 Definition "__init__" is identical in multiple sys.version_info/sys.platform branches
> ./stdlib/pathlib/__init__.pyi:158:9: Y069 Definition "__new__" is identical in multiple sys.version_info/sys.platform branches
> ./stdlib/sysconfig.pyi:52:5: Y069 Definition "is_python_build" is identical in multiple sys.version_info/sys.platform branches
> ./stdlib/tkinter/__init__.pyi:792:9: Y069 Definition "wm_attributes" is identical in multiple sys.version_info/sys.platform branches
> ./stdlib/tkinter/__init__.pyi:823:13: Y069 Definition "wm_attributes" is identical in multiple sys.version_info/sys.platform branches
> ./stubs/pyinstaller/PyInstaller/building/api.pyi:33:5: Y069 Definition "_IconParam" is identical in multiple sys.version_info/sys.platform branches

[srittau]

While I think this is useful to run from time to time, considering the substantial amount of false positives, I suggest to move this to the "not enabled by default" section of error codes.