Add Ruby 3.3 to CI matrix (#53)

Author: m-nakamura145

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: [2.6, 2.7, '3.0', 3.1, 3.2, jruby-9.3, ruby-head, jruby-head]
+        ruby-version: [2.6, 2.7, '3.0', 3.1, 3.2, 3.3, jruby-9.3, ruby-head, jruby-head]
 
     name: Specs - Ruby ${{ matrix.ruby-version }}
     steps: