Add six as an external dep.

BUILD

@@ -510,6 +510,7 @@ py_proto_library(
     include = "src",
     protoc = ":protoc",
     py_extra_srcs = [":python_srcs"],
+    py_libs = ["//external:six"],
     visibility = ["//visibility:public"],
 )
 

WORKSPACE

@@ -5,6 +5,14 @@ new_http_archive(
   build_file = "gmock.BUILD",
 )
 
+new_http_archive(
+  name = "six_archive",
+  url = "https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz#md5=34eed507548117b2ab523ab14b2f8b55",
+  sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
+  build_file = "six.BUILD",
+  strip_prefix = "six-1.10.0",
+)
+
 bind(
   name = "gtest",
   actual = "@gmock_archive//:gtest",
@@ -14,3 +22,8 @@ bind(
   name = "gtest_main",
   actual = "@gmock_archive//:gtest_main",
 )
+
+bind(
+  name = "six",
+  actual = "@six_archive//:six",
+)

six.BUILD

+py_library(
+  name = "six",
+  srcs = ["six-1.10.0/six.py"],
+  visibility = ["//visibility:public"],
+)