Optimize `pathlib.PurePath.__fspath__()`

Feature or enhancement

Return an unnormalized path from pathlib.PurePath.__fspath__()

Pitch

Code like open(Path('./README.txt')) or Path('/home//barney').iterdir() shouldn't require us to normalize the path in pathlib (e.g. remove . segments, doubled slashes, etc), as OS APIs are perfectly happy with unnormalized paths.

We can improve the performance of most Path methods, and the effective performance of passing a Path object to any API that accepts os.PathLike, by skipping normalization in __fspath__().

Prerequisites

Pathlib must not normalize paths on construction:

• Optimize pathlib path construction #101362

Pathlib's normalization must not change the meaning of paths:

• pathlib strips trailing slash #65238
• [pathlib] issues with Windows device paths #78079

Previous discussion

• GH-101362: Optimise pathlib by deferring path normalisation #101560

Linked PRs

• [WIP] GH-102783: Speed up pathlib.PurePath.__fspath__() by returning raw path #112397