Use setup-ocaml@v3

Main contribution of #449.

Resolves #449.

Author: smorimoto

.github/workflows/test.yml

@@ -56,7 +56,7 @@ jobs:
     runs-on: ${{matrix.os}}
     steps:
     - uses: actions/checkout@v2
-    - uses: avsm/setup-ocaml@v2
+    - uses: avsm/setup-ocaml@v3
       with:
         ocaml-compiler: ${{matrix.ocaml}}