The upstream (github) tag tarball is used instead of the pypi one, since the
latter omits the test suite and changelog.

The source is repacked to remove:

 * a vendored copy of the PLY library
 * a compressed copy of the yes(1) manpage (used for a test)
 * images and video without source in the documentation
 * .gitignore (since it interferes with git-buildpackage)

After 0.5, xonsh requires access to the pty device for any operations with
subshells. This can be a problem in build/test chroots (see #817236). The
autopkgtest scripts test whether they can open a pty and avoid some tests
as appropriate.
