add stretch lists / preferences

preferences.d/prefer-stretch.pref

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+# target-release priorities should be >= 990 (and < 1000) apart from backports
+
+# Debian 9
+Package: *
+Pin: release o=Debian,n=stretch*
+Pin-Priority: 990
+
+Package: *
+Pin: release o=Debian Backports,n=stretch-backports
+Pin-Priority: 980
+
+# Debian 10
+Package: *
+Pin: release o=Debian,n=buster*
+Pin-Priority: 890
+
+Package: *
+Pin: release o=Debian Backports,n=buster-backports
+Pin-Priority: 880
+
+# Debian unstable
+Package: *
+Pin: release o=Debian,n=sid*
+Pin-Priority: 300
+
+# Debian others (experimental, ...)
+Package: *
+Pin: release o=Debian
+Pin-Priority: -1

sources.list.d/stretch-src.list

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+deb-src  http://ftp.de.debian.org./debian   stretch                  main contrib non-free
+deb-src  http://ftp.de.debian.org./debian   stretch-updates          main contrib non-free
+deb-src  http://ftp.de.debian.org./debian   stretch-proposed-updates main contrib non-free
+deb-src  http://security.debian.org./       stretch/updates          main contrib non-free
+deb-src  http://ftp.de.debian.org./debian   stretch-backports        main contrib non-free

sources.list.d/stretch.list

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+deb      http://ftp.de.debian.org./debian   stretch                  main contrib non-free
+deb      http://ftp.de.debian.org./debian   stretch-updates          main contrib non-free
+deb      http://ftp.de.debian.org./debian   stretch-proposed-updates main contrib non-free
+deb      http://security.debian.org./       stretch/updates          main contrib non-free
+deb      http://ftp.de.debian.org./debian   stretch-backports        main contrib non-free