shells/xonsh: Upgrade to version 0.7.8

Changelog: https://github.com/xonsh/xonsh/blob/0.7.8/CHANGELOG.rst

PR: 230369
Submitted by: Roberto Fernandez Cueto <roberfern@gmail.com> (maintainer)
Reported by: Nathan <ndowens@yahoo.com>
Reviewed by: krion, mat
Reviewed by: Nathan <ndowens@yahoo.com> (previous version)
Reviewed by: John Hein <z7dr6ut7gs@snkmail.com> (previous version)
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17152