Run CI on Ruby 3.0.

Author: gkellogg

.github/workflows/ci.yml

@@ -31,7 +31,8 @@ jobs:
           - 2.5
           - 2.6
           - 2.7
-          # - ruby-head # net-http-persistent
+          - 3.0
+          - ruby-head
           - jruby
     steps:
       - name: Clone repository