0.24 RC has buggy Python verison select() behavior

[brandjon]

Background

The Python version used for py_binary targets can be affected by multiple command-line flags. In order to give the right answer for select()s that consume the Python version, the target @bazel-tools//tools/python:python_version (and its wrappers, :PY2 and :PY3) mirrors the same logic Bazel uses to decide the Python version. It's important that select() sees the correct version or else action conflicts can arise.

Bug

Bazel 0.24 introduces --incompatible_py3_is_default to change the default Python version. But it did not update the definition of the :python_version target. This means that users could get broken builds when they enable --incompatible_py3_is_default even when the underlying Python targets are properly compatible with Python 3.

The fix is straightforward and low risk for cherrypicking into 0.24.

Repro

# BUILD
sh_binary(
    name = "foo",
    srcs = select({
        "@bazel_tools//tools/python:PY2": ['py2.sh'],
        "@bazel_tools//tools/python:PY3": ['py3.sh'],
    }),
)

# py2.sh
echo "I am PY2!"

# py3.sh
echo "I am PY3!"

$ [bazel 0.24 RC] run :foo
[...]
I am PY2!
 
$ [bazel 0.24 RC] run :foo --python_version=PY3
[...]
I am PY3!
 
# Extra flag needed to pass validation check but otherwise unrelated
$ [bazel 0.24 RC] run :foo --incompatible_py3_is_default --incompatible_allow_python_version_transitions
[...]
I am PY2!

[brandjon]

As per discussion in #6968, this issue can be closed since the code changes are merged in master.