Use cgroups aware processor count by default

In containerized environments, a number of CPU cores isn't the
same as the available CPUs. In this case, we need to consider cgroups.

`concurrent-ruby` now has the method to get that since v1.3.1. I think it's better
to use this for setting more container environment friendly default value.

Ref: ruby-concurrency/concurrent-ruby#1038

Author: y-yagi

lib/parallel.rb

@@ -337,10 +337,10 @@ def physical_processor_count
       end
     end
 
-    # Number of processors seen by the OS, used for process scheduling
+    # Number of processors seen by the OS or value considering CPU quota if the process is inside a cgroup, used for process scheduling
     def processor_count
-      require 'etc'
-      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || Etc.nprocessors)
+      require 'concurrent-ruby'
+      @processor_count ||= Integer(ENV['PARALLEL_PROCESSOR_COUNT'] || Concurrent.available_processor_count.to_i)
     end
 
     def worker_number

parallel.gemspec

@@ -16,5 +16,6 @@ Gem::Specification.new name, Parallel::VERSION do |s|
   }
   s.files = `git ls-files lib MIT-LICENSE.txt`.split("\n")
   s.license = "MIT"
+  s.add_runtime_dependency "concurrent-ruby", "~> 1.0", ">= 1.3.1"
   s.required_ruby_version = '>= 2.7'
 end

spec/parallel_spec.rb

@@ -49,11 +49,6 @@ def without_ractor_warning(out)
         (1..999).should include(Parallel.processor_count)
       end
     end
-
-    it 'uses Etc.nprocessors in Ruby 2.2+' do
-      defined?(Etc).should == "constant"
-      Etc.respond_to?(:nprocessors).should == true
-    end
   end
 
   describe ".physical_processor_count" do