Use py_runtime and python_top while waiting for rules_python.

Test: b run //build/soong/cc/ndkstubgen uses py3cmd as sys.executable
Change-Id: I56341d418b945a0891f18d09670e9a37b1b2ccb3
diff --git a/BUILD.bazel b/BUILD.bazel
index 9e909cc..3b2a41e 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -8,3 +8,13 @@
         "%s/lib64/*" % platform,
     ]),
 ) for platform in ("linux-x86", "darwin")]
+
+
+# TODO(b/199038020): Temporary solution. Use py_runtime_pair and
+# python_toolchain as soon as Python rules are checked-in.
+py_runtime(
+    name = "python3",
+    files = ["linux-x86/bin/py3-cmd"],
+    interpreter = "linux-x86/bin/py3-cmd",
+    python_version = "PY3",
+)