Fix supported Ruby version to be only 3.1.x (#32)

* Update spec.required_ruby_version = "~> 3.1.0"

* Don't run ci test with ruby 3.0 which is not supported

.github/workflows/ci.yaml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        ruby: [3.0, 3.1]
+        ruby: ['3.1']
     runs-on: ${{ matrix.os }}
     steps:
       - uses: ruby/setup-ruby@v1

async_scheduler.gemspec

@@ -12,7 +12,11 @@ Gem::Specification.new do |spec|
   spec.description = "This is a task scheduler which implements the FiberScheduler interface"
   spec.homepage = "https://github.com/kudojp/async_scheduler"
   spec.license = "MIT"
-  spec.required_ruby_version = ">= 3.0.0"
+  # The interface of FiberScheduler#io_read had a breaking change from Ruby 3.0 to 3.1.
+  # - Ruby 3.0: #io_read takes 4 arguments.
+  # - Ruby 3.1: #io_read takes 3 arguments.
+  # The implementation of scheduler#io_read in this gem takes only 3 arguments.
+  spec.required_ruby_version = "~> 3.1.0"
 
   spec.metadata["allowed_push_host"] = "TODO: Set to your gem server 'https://example.com'"