Source python toolchain from rules_nixpkgs.

BUILD

@@ -27,3 +27,32 @@ sh_test(
         ".bazelversion",
     ],
 )
+
+load("@rules_python//python:defs.bzl", "py_runtime_pair")
+
+py_runtime(
+  name = "python2_runtime",
+  python_version = "PY2",
+  files = [],
+  interpreter = "@python//:bin/python",
+)
+
+py_runtime(
+  name = "python3_runtime",
+  python_version = "PY3",
+  files = [],
+  interpreter = "@python3//:bin/python",
+)
+
+py_runtime_pair(
+  name = "my_python_pairs",
+  py2_runtime = ":python2_runtime",
+  py3_runtime = ":python3_runtime",
+)
+
+toolchain(
+  name = "py_pair_toolchain",
+  #target_compatible_with = <...>,
+  toolchain = ":my_python_pairs",
+  toolchain_type = "@rules_python//python:toolchain_type",
+)

WORKSPACE

@@ -30,6 +30,16 @@ nixpkgs_package(
   repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
 )
 
+nixpkgs_package(
+  name = "python",
+  repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
+)
+
+nixpkgs_package(
+  name = "python3",
+  repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
+)
+
 nixpkgs_package(
   name = "flex",
   repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
@@ -108,6 +118,8 @@ load("//:setup.bzl", "kythe_rule_repositories", "maybe")
 # NOTE(treetide): local modification, we hardwired the nix-based values here.
 register_toolchains("//tools/build_rules/lexyacc:lexyacc_local_toolchain")
 
+register_toolchains("//:py_pair_toolchain")
+
 kythe_rule_repositories()
 
 # NOTE(treetide): local