Use python3 package in the build using Alpine

The package has recently been renamed from python to python3

Author: marco-c

.taskcluster.yml

@@ -328,7 +328,7 @@ tasks:
                 command:
                   - sh
                   - -lxce
-                  - "apk add --quiet --update git python &&
+                  - "apk add --quiet --update git python3 &&
                     git clone --quiet ${repository} /src && cd /src/addon && git checkout ${head_rev} -b build &&
                     npm install --no-progress &&
                     npm run build &&