Deprecate Windows-to-POSIX separator conversion via `PurePosixPath(PureWindowsPath(...)))`

Feature or enhancement

This was probably never an intentional feature, and results in subtle behaviour:

>>> from pathlib import PurePosixPath, PureWindowsPath
>>> from os import fspath
>>> p = PureWindowsPath('foo', 'bar')
>>> PurePosixPath(p)
PurePosixPath('foo/bar')
>>> p = fspath(p)
>>> PurePosixPath(p)
PurePosixPath('foo\\bar')

PurePath is documented as accepting os.PathLike objects, and so the additional os.fspath() call shouldn't really make a difference. See #103631 for why it does (spoiler: backwards-compatibility).

Folks can use PurePosixPath(PureWindowsPath().as_posix()) instead, which makes it more explicit that backwards-to-forwards slash conversion is desired.

Linked PRs

• gh-125863

先檢視 issue 和連結的 PR gh-125863 中所描述的 pathlib PurePath 處理方式。比較 PureWindowsPath 和 PurePosixPath 的範例，接著確認已棄用的轉換行為和明確的替代方案 as_posix() 已涵蓋在產生的變更中。