Build and test on 5.3.0 too

Author: jmid

.github/workflows/main.yml

@@ -22,6 +22,7 @@ jobs:
           - "4.14"
           - "5.0"
           - "5.2"
+          - "5.3"
         exclude:
           - os: windows-latest
             ocaml-compiler: "4.08"