lpath of filesystem.get doesn't handle pathlib.Path gracefully

I didn't find an existing issue, so I am opening a new one.

It looks like we can't pass pathlib Path's as local paths for get. I've only tried it against the GitHub implementation so far but considering that it fails in what appears to be a general utility function it might affect other implementations, too. Here is a reproducing example of the problem:

>>> import fsspec
>>> from pathlib import Path
>>> foo = Path("test_file.md") 
>>> fs = fsspec.filesystem("github", org="fsspec", repo="filesystem_spec")
>>> fs.get("README.md", foo)      
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\fsspec\spec.py", line 771, in get
    lpaths = other_paths(rpaths, lpath)
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\fsspec\utils.py", line 380, in other_paths
    assert len(paths) == len(path2)
TypeError: object of type 'WindowsPath' has no len()
>>> fs.get("README.md", str(foo))  # this works fine
>>>

Would it be possible to allow inputs of type pathlib.path for lpath? (e.g., by simply converting them to posix path strings: lpath.as_posix())

Start with get in fsspec/spec.py and the other_paths utility in fsspec/utils.py, using the pathlib.Path reproduction from the issue. Verify that a Path can be used as lpath while the existing string input continues to work; completion is demonstrated when the reproduction no longer raises the WindowsPath len() TypeError.