Make python3 command on Linux point to Python 3.8.0

Author: JSAbrahams

.travis.yml

@@ -49,6 +49,8 @@ before_install:
       sudo add-apt-repository -y ppa:deadsnakes/ppa;
       sudo apt-get update;
       sudo apt-get install python3.8;
+      sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8;
+      sudo update-alternatives --config python3;
     fi
 
   # grcov