Use alpine:3.15 in python_syntax

ci-templates/syntax-check.yml

@@ -20,6 +20,7 @@ clang_format:
 ## other options like pylint, but much faster and less strict
 ##########################################################
 python_syntax:
+  image: alpine:3.15 # python2 is no longer available in alpine:3.16
   before_script:
     - apk add --update python2
   stage: build