packaging: follow hg's supported python version (>= 2.7) "Mercurial 4.3 and newer require Python 2.7." (From https://www.mercurial-scm.org/wiki/SupportedPythonVersions) Also add X- prefix, because that's the correct form, apparently. This line can also be removed in future, since "When Debian supported multiple Python versions, X-Python-Version was used, but it is obsolete now as no supported Debian release supports anything other than python2.7." (From https://wiki.debian.org/Python/LibraryStyleGuide) That page also mentions "X-Python3-Version".