PurePath.with_suffix(suffix) 
Return a new path with the suffix changed. If the original path doesn’t have a suffix, the new suffix is appended instead:
>>> p = PureWindowsPath('c:/Downloads/pathlib.tar.gz')
>>> p.with_suffix('.bz2')
PureWindowsPath('c:/Downloads/pathlib.tar.bz2')
>>> p = PureWindowsPath('README')
>>> p.with_suffix('.txt')
PureWindowsPath('README.txt')
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