Add Ruby 3.0 support

Author: HarlemSquirrel

.github/workflows/test.yml

@@ -22,6 +22,7 @@ jobs:
           - "2.5"
           - "2.6"
           - "2.7"
+          - "3.0"
           - "jruby-9.2"
     steps:
     - uses: actions/checkout@v2