Stubs for filecmp are missing Pathlike arguments

[BoboTiG]

(Inspired from #2267)

Starting with Python 3.6, filecmp functions can handle path-like objects, including:

filecmp.cmp()
filecmp.cmpfiles()
filecmp.dircmp()

Reproduction:

import filecmp
from pathlib import Path

filecmp.dircmp(Path("/tmp"), "/tmp")
filecmp.cmp(Path("file1"), Path("file2"))

Will output:

error: Value of type variable "AnyStr" of "dircmp" cannot be "Path"
error: Argument 1 to "cmp" has incompatible type "Path"; expected "Union[bytes, str]"
error: Argument 2 to "cmp" has incompatible type "Path"; expected "Union[bytes, str]"

I will not have time to work on it, but I think it is a good thing to have a open issue for anyone wanting to give a hand.

[djb4ai]

@srittau Opened a PR. Let me know any changes, thanks.