file_util: improve rmtree performance #1672

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The following news fragments found: [<PatchedFile: releng/release-notes-next/improve-file_util_rmtree-performance.bugfix>]

Pattern: re.compile('releng/release-notes-next/(?P<issue_number>[^\\./]+)\.(?P<fragment_type>breaking|feature|bugfix|config)(\.\d+)?(\.[^\\./]+)*$')