Travis: install venv

.travis.yml

@@ -12,6 +12,7 @@ before_install:
     - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
     - sudo apt-get update -q
     - sudo apt-get install -y gcc-arm-embedded
+    - sudo apt-get install -y python3-venv
 script:
   - export CC=gcc-7
   - make travis