#6660, #665, and #6418 added support for pathlib.Path in numpy.load.

However, if mmap_mode is not None in numpy.load, I get an error:

  File ".../python3.6/site-packages/numpy/lib/format.py", line 747, in open_memmap
    raise ValueError("Filename must be a string.  Memmap cannot use"
ValueError: Filename must be a string.  Memmap cannot use existing file handles.

The problem is that format.open_memmap does not yet support pathlib.Path.

https://github.com/numpy/numpy/blob/master/numpy/lib/format.py#L703
https://github.com/numpy/numpy/blob/master/numpy/lib/npyio.py#L430

Please let me know whether you would accept a PR for this.