python_dependency: move bottle check into method.

Library/Homebrew/requirements/python_dependency.rb

@@ -7,7 +7,7 @@ class PythonDependency < Requirement
   satisfy :build_env => false do
     # Always build bottles against Homebrew Python as they still work
     # against a 2.7 system Python.
-    next if ARGV.build_bottle?
+    next if build_bottle?
     python = which_python
     next unless python
     version = python_short_version
@@ -27,6 +27,10 @@ class PythonDependency < Requirement
     end
   end
 
+  def build_bottle?
+    ARGV.build_bottle?
+  end
+
   def python_short_version
     @short_version ||= Language::Python.major_minor_version which_python
   end