Amend python pkgs for debian-based distros

Drop python2 packages that are no longer needed. Replace other py2 pkgs
with their py3 variant.

Change-Id: Id5fc11d1ae0a7daf439c7d72f831f298f5d0ce31
diff --git a/files/debs/n-cpu b/files/debs/n-cpu
index 636644f..54d6fa3 100644
--- a/files/debs/n-cpu
+++ b/files/debs/n-cpu
@@ -5,7 +5,7 @@
 lvm2 # NOPRIME
 netcat-openbsd
 open-iscsi
-python-guestfs # NOPRIME
+python3-guestfs # NOPRIME
 qemu-utils
 sg3-utils
 sysfsutils