Load @rules_python in six.BUILD

[Yannic]

This PR adds a load statement for py_library to make @six compatible with bazelbuild/bazel#9006.

//cc @aaliddell @gnossen

[aaliddell]

If I remember correctly, there was a reason it was done this way. I'll have a check

[aaliddell]

See #6391

Prior to that PR, it was done effectively as you are now in this PR (i.e the only file in the library is six.py). However, we can't know at the @six//:six py_library creation time what the legacy_create_init flag might be set to when anyone creates a py_binary that depends on six. Since a number of other repos depend on @six, we can't just fix all the py_binary definitions in protobuf. Therefore, by moving six.py to __init__.py, we force it to work regardless of the legacy_create_init flag, as it won't overwrite an existing __init__.py file.

At some point legacy_create_init is getting flipped and removed, at which point this bodge can be fixed.

Edit: In hindsight, I should have annotated the code with this detail at the time...

[Yannic]

@aaliddell Thank you! Also fixed this in my PR for grpc.

Updated the PR to add the missing load for @rules_python. @acozzette can you review?

[acozzette]

Thanks, @Yannic.