Toolchains with Bazel 0.5.4

Question

Is there an import of rules_python that defines toolchains but is compatible with bazel release 0.5.4? If not, what's the minimum version of bazel that does implement toolchains?

$bazel info release
release 0.5.4

I've inherited an application that builds with bazel 0.5.4, and would prefer not to destabilize the application with a significant upgrade. But, it does require toolchains in order to find python 3.

Suppose the application consists of the WORKSPACE and BUILD files from the most recent rules release README, plus a small python objective in the BUILD:

py_test(
    name = "sandbox_test",
    srcs = ["sandbox_test.py"],

    default_python_version = "PY3",
    srcs_version = "PY3",
)

What additional WORKSPACE definitions could enable this to run? With the latest rules_python provided:

git_repository(
    name = "rules_python",
    commit = "740825b7f74930c62f44af95c9a4c1bd428d2c53",
    remote = "https://github.com/bazelbuild/rules_python.git",
)

toolchain.bzl cannot be found:

ERROR: error loading package 'toolchain_demo': Extension file not found. Unable to load file '@bazel_tools//tools/python:toolchain.bzl': file doesn't exist or isn't a file
ERROR: error loading package 'toolchain_demo': Extension file not found. Unable to load file '@bazel_tools//tools/python:toolchain.bzl': file doesn't exist or isn't a file

Is there a way to patch toolchain definitions into the WORKSPACE, or are toolchains with 0.5.4 impossible?