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Python versions, X-Python-Version was used, but it is obsolete now as no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".