CI-unixish-docker.yml: install the python3 package for ubuntu instead

Author: firewave

.github/workflows/CI-unixish-docker.yml

@@ -30,7 +30,7 @@ jobs:
         if: matrix.image != 'centos:7'
         run: |
           apt-get update
-          apt-get install -y cmake g++ make python libxml2-utils
+          apt-get install -y cmake g++ make python3 libxml2-utils
           apt-get install -y libpcre3-dev
 
       # tests require CMake 3.4