tarball: extract safemembers() from within _uncompress_tar()

No need for it being a dynamically defined function, plus it makes pydocstyle very unhappy (probably a bug in pydoctyle 4.0.0 but...).

Also rename it as _safemembers() for consistency sake.

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Migrated from D1706 (view on Phabricator)

Build is green See https://jenkins.softwareheritage.org/job/DCORE/job/tox/248/ for more details.

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