Support pathlib.Path instances for file-type input

[huard]

Description

Reference to input files should be able to accept pathlib.Path objects.

[huard]

The issue occurs when the server is localhost, and we don't need to embed the resource. The path is passed to birdy,utils.fix_url

def fix_url(url):
    """If url is a local path, add a file:// scheme."""
    return urlparse(url, scheme='file').geturl()

which does not support Path objects.