For brewed python, make sure to unset the PYTHONPATH

during building of python software inside of a `python do … end` block.

Library/Homebrew/requirements/python_dependency.rb

@@ -259,7 +259,7 @@ class PythonInstalled < Requirement
     ENV.prepend 'PATH', binary.dirname, ':' unless from_osx?
 
     ENV['PYTHONHOME'] = nil  # to avoid fuck-ups.
-    ENV['PYTHONPATH'] = global_site_packages.to_s unless brewed?
+    ENV['PYTHONPATH'] = if brewed? then nil; else global_site_packages.to_s; end
     ENV.append 'CMAKE_INCLUDE_PATH', incdir, ':'
     ENV.append 'PKG_CONFIG_PATH', pkg_config_path, ':' if pkg_config_path
     # We don't set the -F#{framework} here, because if Python 2.x and 3.x are