Breaks with Python toolchains (Bazel 0.27)

[brandjon]

Bazel 0.27 flips --incompatible_use_python_toolchains, which breaks this repo. Note that this pipeline was already failing due to a different error.

This flag causes Bazel to use the version of Python requested at analysis time, rather than whatever the system python command happens to be. You likely need to add python_version = "PY2" to your Python targets, and/or enable --force_host_python=PY2 in your bazelrc (if any of these are used in the host configuration).