Add pathlib.Path support to sox_io backend

[timlod]

Allows for passing of pathlib.Path filepath to sox_io functions.
See #902.

For type union docstrings I used (str/pathlib.Path), as I found this pattern was used in the kaldi_io methods (for str/FileDescriptor).
There may be a better pattern though?

[vincentqb]

Thanks for following-up on comment :)

torchscript tests are failing, and indicate that the union type is not supported by torchscript, see also here. For soundfile, even though functions convert to string and make the code compatible with path, only the string type is in the signature, here. I would be ok having a similar unofficial support for pathlib as is done in soundfile here.

[mthrok]

Hi @timlod

Thanks for the PR. The signature part is a tricky one, which had slipped from my mind when I was replying in #902.

The "sox_io" backend is designed to be compatible with TorchScript compiler (so that the function works in C++ application too), so types that are specific to Python (in this case, they are Union, Path) cannot be added. Also, because of the way the loading function (in C++) is bound to Python code, we cannot pass file-like object, even though that's common practice in Python.

I checked the intermediate representation that TorchScript compiler generates and filepath = str(filepath) has no effect in TorchScript so this change is good. So just keeping the signature as is should do.

[mthrok]

Hi @timlod

Thanks for updating the PR. Changes related to the other backends can be omitted from this PR.

[mthrok]

Looks good, thanks!