Add CcInfo to allowed providers of py_library deps.

This is required in order to use a
[pybind_extension](https://github.com/pybind/pybind11_bazel/blob/2b6082a4d9d163a52299718113fa41e4b7978db5/build_defs.bzl#L28)
in the `deps` of a `py_libary` as the `pybind_extension` is just a
`cc_binary` under the hood.

The `py_libary` from the official `rules_python` also supports this.

Author: lgulich

py/private/py_library.bzl

@@ -205,7 +205,7 @@ _attrs = dict({
     ),
     "deps": attr.label_list(
         doc = "Targets that produce Python code, commonly `py_library` rules.",
-        providers = [[PyInfo], [PyVirtualInfo]],
+        providers = [[PyInfo], [PyVirtualInfo], [CcInfo]],
     ),
     "data": attr.label_list(
         doc = """Runtime dependencies of the program.