Merge pull request #83 from dolmen-go/ci-add-go1.20

CI: add Go 1.20

Author: mattn

.github/workflows/test.yml

@@ -14,8 +14,9 @@ jobs:
           - macos-latest
           - windows-latest
         go:
-          - '1.18'
+          - '1.20'
           - '1.19'
+          - '1.18'
           - '1.17'
           - '1.16'
           - '1.15'