Fix tests for --incompatible_use_python_toolchains

[brandjon]

This adds a hook file for run_tests.sh to write toolchain info to before each bazel invocation. This replaces the legacy way of passing an interpreter in via --python_path.

It also replaces a config_setting that was used to control whether PY2 or PY3 was used, with a simple constant symbol consumed at loading time. This is needed in order to make the target aware at analysis time of which version it is building for.

Fixes #98, fixes #102.

[brandjon]

@aaliddell, @tmc, I don't think github's UI will let me add you as reviewers since you don't have write access, but I would appreciate comments before I merge.

[tmc]

This seems reasonable to me.

[brandjon]

Thanks, merging.