GH-73991: Rework pathlib.Path.copytree() into copy()

[barneygale]

Rename pathlib.Path.copy() to _copy_file() (i.e. make it private.)

Rename pathlib.Path.copytree() to copy(), and add support for copying non-directories. This simplifies the interface for users, and nicely complements the upcoming move() and delete() methods (which will also accept any type of file.)

• Issue: Support moving across filesystems in pathlib.Path, as shutil.move() does #73991

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📚 Documentation preview 📚: https://cpython-previews--122369.org.readthedocs.build/

[barneygale]

Making the old copy() method private was a bit of a can of worms, as it made our usage of _winapi.CopyFile2 simpler, which in turn removed any need for UnsupportedOperation to be defined in _os.py rather than _abc.py.

[barneygale]

Hey @nineteendo, could you review this please, if you're up for it? Thanks in advance!

[AA-Turner]

Danke!

A

[barneygale]

Thank you for the review, very helpful!