Fix the open-uri test to work on all versions

* open-uri no longer makes #open open URLs on MRI 2.8 dev

.github/workflows/test.yml

@@ -63,7 +63,7 @@ jobs:
     - name: OpenSSL version
       run: ruby -ropenssl -e 'puts OpenSSL::OPENSSL_LIBRARY_VERSION'
     - name: OpenSSL test
-      run: ruby -ropen-uri -e 'puts open(%{https://rubygems.org/}) { |f| f.read(1024) }'
+      run: ruby -ropen-uri -e 'puts URI.send(:open, "https://rubygems.org/") { |f| f.read(1024) }'
 
     - name: C extension test
       run: gem install json:2.2.0 --no-document