shutil: Inconsistent return types when using pathlib

Bug report

Bug description:

Using pathlib with shutil usually works, but there are some quirks when it comes to return types. As an example, see the copy function (but other functions are affected as well):

https://github.com/python/cpython/blob/67ded6a4faae29edff8e4f7886978e71ce116e33/Lib/shutil.py#L468-L484

In this case, if a Path object gets passed in as dst, it will be returned unchanged if it is not a directory, but a str will be returned if it is a directory. That's unexpected, the return type should be consistent, probably by always returning a str.

I assume that pathlib support is mostly incidental in shutil, considering that the latter predates the former. shutil should probably be reviewed for proper pathlib support at some point. (See also a few other pathlib/shutil related issues reported here.)

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-132820

Check linked PR gh-132820 first, then inspect Lib/shutil.py around copy and the other pathlib-related functions mentioned by the report. Review existing shutil tests and ensure the affected functions have consistent, documented return types, with tests covering Path destinations that are and are not directories.