Run compilation tests against Ruby 3.3.0-preview2

Before precompiling the native gems against the latest, unreleased
version of Ruby, check the gem compiles and passes its tests when the
user has to compile against it on install.

Note preview versions of Ruby are not available for Windows, see
https://github.com/ruby/setup-ruby#supported-versions

See #108

.github/workflows/tests.yml

@@ -70,6 +70,7 @@ jobs:
     strategy:
       matrix:
         ruby:
+          - '3.3'
           - '3.2'
           - '3.1'
           - '3.0'
@@ -124,7 +125,7 @@ jobs:
       fail-fast: false
       matrix:
         runs-on: ["ubuntu-latest", "macos-latest"]
-        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2"]
+        ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3"]
         include:
           - ruby: "2.6"
             runs-on: "windows-2019"