Change to debian non-native (quilt) format

The debian release is only one of many releases, thus non-native
makes more sense

Further details at:
- https://wiki.debian.org/DebianMentorsFaq#What_is_the_difference_between_a_native_Debian_package_and_a_non-native_package.3F
- https://wiki.debian.org/Packaging/Intro#The_packaging_work_flow

Debian build instructions for upstream release 0.11.3
-----------------------------------------------------
```
  # Assuming sbuild is installed and chroot is created
  # https://wiki.debian.org/sbuild

  # Download upstream tarball + signature and rename to expected pattern
  wget https://github.com/secure-systems-lab/securesystemslib/releases/download/sslibv0.11.3/securesystemslib-0.11.3.tar.gz \
      -O python-securesystemslib_0.11.3.orig.tar.gz
  wget https://github.com/secure-systems-lab/securesystemslib/releases/download/sslibv0.11.3/securesystemslib-0.11.3.tar.gz.asc \
      -O python-securesystemslib_0.11.3.orig.tar.gz.asc

  # Extract upstream tarball
  tar xf python-securesystemslib_0.11.3.orig.tar.gz

  # Clone upstream repo for debian files and tests
  git clone https://github.com/secure-systems-lab/securesystemslib.git \
      securesystemslib-upstream

  # Copy debian files (from debian branch)
  git -C securesystemslib-upstream checkout debian --
  cp -r securesystemslib-upstream/debian securesystemslib-0.11.3/

  # Change to unpacked tarball
  cd securesystemslib-0.11.3

  ##################
  # Optionally copy and run tests (from corresponding release tag)
  # NOTE: The next upstream release will include tests in the source tarball
  # see #168
  git -C ../securesystemslib-upstream checkout sslibv0.11.3
  cp -r ../securesystemslib-upstream/tests .
  # FIXME: Sbuild will complain about local changes and abort building. One way
  # to solve this is to create downstream patches, but there must be a better way
  # to only run tests in chroot as sbuild would do it, without including the
  # tests to the downstream release.
  dpkg-source --commit
  sbuild -A -s -d unstable
  ##################

  # Build (w/o tests)
  DEB_BUILD_OPTIONS=nocheck sbuild -A -s -d unstable

  # Sign debian files (replace keyid if necessary)
  debsign -k 8BA69B87D43BE294F23E812089A2AD3C07D962E8
```

debian/changelog

@@ -1,4 +1,4 @@
-python-securesystemslib (0.11.3) unstable; urgency=low
+python-securesystemslib (0.11.3-1) unstable; urgency=low
 
   * Initial Debian release from tag:
 

debian/source/format

@@ -1 +1 @@
-3.0 (native)
+3.0 (quilt)