Enable Starlark `py_runtime`.
Enable Starlark `py_runtime`.

I manually built a Bazel and ran tests in rules_python with it; looks to be OK.

Work towards #15897

PiperOrigin-RevId: 485978566
Change-Id: I79c209b095492c39b1fff97091425fc7af486b12

bazelbuild/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,bazelbuild/bazel,katre/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,bazelbuild/bazel