[Python] Support pathlib.Path arguments for ExecutionContext.register_* methods

[kszucs]

Currently it expects a python string:

    def test_register(ctx, tmp_path):
        path = write_parquet(tmp_path / "a.parquet", data())
>       ctx.register_parquet("t", path)
E       TypeError: argument 'path': 'PosixPath' object cannot be converted to 'PyString'

[drauschenbach]

Obsolete, now that datafusion-python is an external git repo.

[andygrove]

Closing this. Thanks @drauschenbach.