build(python): align toolchain with monorepo pattern

bazel/python.BUILD.template

@@ -0,0 +1,25 @@
+load("@rules_python//python:py_runtime.bzl", "py_runtime")
+load("@rules_python//python:py_runtime_pair.bzl", "py_runtime_pair")
+
+py_runtime(
+    name = "runtime",
+    implementation_name = "cpython",
+    interpreter_path = "{interpreter_path}",
+    interpreter_version_info = {
+        "major": "{major}",
+        "minor": "{minor}",
+        "micro": "{micro}",
+    },
+    python_version = "PY3",
+)
+
+py_runtime_pair(
+    name = "runtime_pair",
+    py3_runtime = ":runtime",
+)
+
+toolchain(
+    name = "python",
+    toolchain = ":runtime_pair",
+    toolchain_type = "@rules_python//python:toolchain_type",
+)