Add Language::Python module.

Library/Homebrew/language/python.rb

@@ -0,0 +1,26 @@
+require "utils.rb"
+
+module Language
+  module Python
+    def self.major_minor_version python
+      version = /\d\.\d/.match `#{python} --version 2>&1`
+      return unless version
+      Version.new(version.to_s)
+    end
+
+    def self.each_python build, &block
+      original_pythonpath = ENV["PYTHONPATH"]
+      ["python", "python3"].each do |python|
+        next if build.without? python
+        version = self.major_minor_version python
+        ENV["PYTHONPATH"] = if Formula.factory(python).installed?
+          nil
+        else
+          "#{HOMEBREW_PREFIX}/lib/python#{version}/site-packages"
+        end
+        block.call python, version if block
+      end
+      ENV["PYTHONPATH"] = original_pythonpath
+    end
+  end
+end