cmp functions have been removed in Python 3 and are slower.

Use debian.debian_support.Version wrapper class for key functions
as apt_pkg doesn't appear to have an appropriate class.

Thanks-to: Stuart Prescott <stuart@debian.org>

Traceback (most recent call last):
  File "bin/compare-source-package-list", line 1186, in <module>
    files, patches, links, new = process_sources(source_entries, lists_dir)
  File "bin/compare-source-package-list", line 1164, in process_sources
    actions = check_source_package(source_entry, srcpkg)
  File "bin/compare-source-package-list", line 1113, in check_source_package
    derived_from = find_derived_from(tmp_dir, name, version, dsc_name, dsc_sha1, parts_unmodified)
  File "bin/compare-source-package-list", line 885, in find_derived_from
    possibly_derived_from.sort(cmp=lambda a,b: apt_version_cmp(b[1],a[1]))
TypeError: 'cmp' is an invalid keyword argument for this function

Traceback (most recent call last):
  File "bin/compare-source-package-list", line 1245, in <module>
    data.sort(cmp=lambda a,b: cmp(a[0],b[0]) or apt_version_cmp(a[1],b[1]) or cmp(a[2],b[2]) or apt_version_cmp(a[3],b[3]))
TypeError: 'cmp' is an invalid keyword argument for this function

Traceback (most recent call last):
  File "bin/compare-source-package-list", line 1266, in <module>
    data.sort(cmp=lambda a,b: cmp(a[0],b[0]) or apt_version_cmp(a[1],b[1]) or cmp(a[2],b[2]))
TypeError: 'cmp' is an invalid keyword argument for this function

Prevents crashes from incompatible types.
Prevents printing Python bytes literals in logs.

The code now works with python3.

The default is now to restore default signal handlers in sub-processes.