metadata: use python-xattr pyxattr compat code

metadata: use python-xattr pyxattr compat code

Newer versions of python-xattr have pyxattr compatibility code
(since my pull request https://github.com/xattr/xattr/pull/38).

Modify bup to allow using it; prefer pyxattr since it's faster,
but sometimes due to other dependencies python-xattr might be
present already (and since it conflicts with pyxattr, it's not
possible to install both.)

Also add a .cirrus.yml task to test this on Debian.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>