pathlib ABCs: check filename validity when copying #139005
                
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Use
target_path.joinpath('_').with_name(source_name)to generate child paths in a target directory when copying. This formula ensuresValueErroris raised if the filename is invalid for the target path type (e.g. empty or contains a path separator.)No change of public behaviour because
pathlib.Pathis the only exposed implementation ofReadablePath, and validity issues arise only when the source and target path parsers differ.