[Feature request] Add PosixPath object support for python r2pipe

[dukebarman]

In Python3 if you use pathlib module for parsing files from a directory, you get results in PosixPath objects. When you send it to r2 = r2pipe.open(filename), you receive an error:

File "main.py", line 30, in is_binary
    r2 = r2pipe.open(filename)
  File "/usr/local/lib/python3.7/site-packages/r2pipe/open_sync.py", line 32, in __init__
    super(open, self).__init__(filename, flags)
  File "/usr/local/lib/python3.7/site-packages/r2pipe/open_base.py", line 132, in __init__
    if filename.startswith("#!pipe"):
AttributeError: 'PosixPath' object has no attribute 'startswith'

And you need convert this path to string: r2 = r2pipe.open(str(filename)). It would be good add support or check this functionality in r2pipe python module.