[bazel] Add `strip_prefix` to download of @six

This way, we can avoid an unnecessary copy.

protobuf_deps.bzl

@@ -27,6 +27,7 @@ def protobuf_deps():
             name = "six",
             build_file = "@com_google_protobuf//:third_party/six.BUILD",
             sha256 = "d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73",
+            strip_prefix = "six-1.12.0",
             urls = ["https://pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz"],
         )
 

third_party/six.BUILD

@@ -1,13 +1,8 @@
-genrule(
-  name = "copy_six",
-  srcs = ["six-1.12.0/six.py"],
-  outs = ["__init__.py"],
-  cmd = "cp $< $(@)",
-)
+load("@rules_python//python:defs.bzl", "py_library")
 
 py_library(
     name = "six",
-  srcs = ["__init__.py"],
+    srcs = ["six.py"],
     srcs_version = "PY2AND3",
     visibility = ["//visibility:public"],
 )