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.

py/tests/cc-deps/BUILD.bazel

@@ -0,0 +1,15 @@
+load("//py:defs.bzl", "py_test")
+
+# Test that cc targets can be used as deps for py targets.
+# We only add a very simple test to not pull in all the deps
+# to build proper py-bindings.
+cc_library(
+    name = "example_library",
+    srcs = ["example_library.cpp"],
+)
+
+py_test(
+    name = "test_smoke",
+    srcs = ["test_smoke.py"],
+    deps = [":example_library"],
+)

py/tests/cc-deps/example_library.cpp

@@ -0,0 +1,3 @@
+
+int add(int i, int j) { return i + j; }
+

py/tests/cc-deps/test_smoke.py

@@ -0,0 +1,5 @@
+def main():
+    print("Running successfully!")
+
+if __name__ == "__main__":
+    main()