ffmpeg is available on default ubuntu packages now.

anntzer · anntzer · commit 182c0b893f05 · 2019-12-07T12:48:06.000+01:00
... whereas ppa:jonathonf/ffmpeg-3 has gone offline.
diff --git a/.travis.yml b/.travis.yml
@@ -21,8 +21,6 @@ addons:
     paths:
     - result_images.tar.bz2
   apt:
-    sources:
-      - sourceline: ppa:jonathonf/ffmpeg-3
     packages:
       - cm-super
       - dvipng
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
@@ -49,7 +49,6 @@ steps:
 - bash: |
     case "$(python -c 'import sys; print(sys.platform)')" in
     linux)
-      sudo apt-add-repository ppa:jonathonf/ffmpeg-3
       sudo apt-get update
       sudo apt-get install \
         cm-super \
