open_filename doesn't accept a pathlib object

[estshorter]

Bug report

If the filename argument is a pathlib object, it is mistakenly recognized as a file handler.
I think open_filename should accept a pathlib object.

pdfminer.six/pdfminer/utils.py

Lines 14 to 25 in 0b44f77

	class open_filename(object):
	"""
	Context manager that allows opening a filename and closes it on exit,
	(just like `open`), but does nothing for file-like objects.
	"""
	def __init__(self, filename, *args, **kwargs):
	if isinstance(filename, str):
	self.file_handler = open(filename, *args, **kwargs)
	self.closing = True
	else:
	self.file_handler = filename
	self.closing = False

[pietermarsman]

Hi @estshorter,

I was hesitant at first, because there might be a lot of other packages that also do some kind of path representation. And they probably all can map it to a string (which we already support). But since pathlib is a python core package we can support it.

When changing this, it should also be more verbose in case of any expected input. So maybe use isinstance(something, io.IOBase) for the file like output, and add one for the pathlib, and if none match raise an error.