Fix the `pybind_library_test` rule.

Specifically, replace dependencies that never existed (at least, not
outside Google) with a dependency on the Python toolchain libraries.

Fixes #36.

build_defs.bzl

diff --git a/build_defs.bzl b/build_defs.bzl
index ee34502..c71642f 100644
--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -92,8 +92,7 @@
         features = features + PYBIND_FEATURES,
         tags = tags,
         deps = deps + PYBIND_DEPS + [
-            "//util/python:python_impl",
-            "//util/python:test_main",
+            "@rules_python//python/cc:current_py_cc_libs",
         ],
         **kwargs
     )