python_dependency: always bottle with brew Python.

Library/Homebrew/requirements/python_dependency.rb

@@ -5,6 +5,9 @@ class PythonDependency < Requirement
   default_formula "python"
 
   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?
     python = which_python
     next unless python
     version = python_short_version