uses `cmp` keyword of `sorted()` which has been removed in Python3

[umlaeute]

as of Python3 (released 2008), the sorted() function no longer has a cmp keyword (see here)

however, in current master it is still used:

python-aptly/aptly/publisher/__init__.py

Line 421 in 0cf9f76

packages = sorted(packages, key=lambda x: self.parse_package_ref(x)[2], reverse=True, cmp=apt_pkg.version_compare)

consequently, the publishing process fail on Python3 systems (and with Python2 being dead dead dead, this means that publishing is broken in general)