GH-73991: Rework `pathlib.Path.copytree()` into `copy()` (#122369)

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.)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>