Add explicit dependency on python3-yaml, which is used in the code but was previously pulled in indirectly via ubuntu-image. LP: #1830823.

debian/changelog

@@ -1,3 +1,10 @@
+livecd-rootfs (2.408.49) UNRELEASED; urgency=medium
+
+  * Add explicit dependency on python3-yaml, which is used in the code but
+    was previously pulled in indirectly via ubuntu-image.  LP: #1830823.
+
+ -- Steve Langasek <steve.langasek@ubuntu.com>  Thu, 30 May 2019 14:17:45 -0700
+
 livecd-rootfs (2.408.48) xenial; urgency=medium
 
   * Do not depend on ubuntu-image on powerpc as it is not a supported platform

debian/control

@@ -29,6 +29,7 @@ Depends: ${misc:Depends},
          procps,
          python-minimal | python,
          python3-software-properties,
+         python3-yaml,
          qemu-utils,
          rsync,
          snapd,