Python `packaging` package is unavailable in older distributions
In https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10922 we migrated from using `distutils.version` in our Sphinx configuration to `packaging.version` as the former is not available in recent Python releases. However, it turns out that the latter is not available in Debian 9 and Ubuntu 18.04. Consequently, we need to support both
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