Hermetic Python 3 test is missing

[abergmeier-dsfishlabs]

Description of the problem / feature request / question:

While Python 2 is getting tested in https://github.com/bazelbuild/bazel/blob/master/src/test/java/com/google/devtools/build/lib/bazel/rules/python/BazelPyRuntimeTest.java, there is no such test for Python 3.
Since Bazel is notorious for breaking Python 3 - please ensure that this gets tested.

[abergmeier-dsfishlabs]

Here is what this leads to: #3568

[brandjon]

I'm not exactly clear on what test coverage this issue is requesting, or that it's specifically a Py 2/3 issue as opposed to just a py_runtime issue.

[brandjon]

Closing this because the problem's unclear.

For py_runtime analysis-time tests we now have PyRuntimeConfiguredTargetTest.

For end-to-end tests we have some attempt to launch both a Python 2 and Python 3 runtime, though the coverage could stand to be improved (#6868). This will wait until at least after the toolchain work is done.