CI: Add Ruby 3.0

Author: mrkn

.github/workflows/ci.yml

@@ -21,11 +21,13 @@ jobs:
           - 2.5
           - 2.6
           - 2.7
+          - 3.0
           - head
         include:
           - { os: windows-latest , ruby: mingw }
           - { os: windows-latest , ruby: mswin }
         exclude:
+          - { os: windows-latest , ruby: 3.0 }
           - { os: windows-latest , ruby: head }
 
     steps: