Consider

    import fileinput
    import pathlib
    with fileinput.input(files=(pathlib.Path('in.txt'),), inplace=True) as fp:
        for line in fp:
            print(line, end='')

which results in

    Traceback (most recent call last):
      File "./pathlib-fileinput.py", line 6, in <module>
        for line in fp:
      File "/Users/zmwang/.pyenv/versions/3.6.1/lib/python3.6/fileinput.py", line 250, in __next__
        line = self._readline()
      File "/Users/zmwang/.pyenv/versions/3.6.1/lib/python3.6/fileinput.py", line 331, in _readline
        self._filename + (self._backup or ".bak"))
    TypeError: unsupported operand type(s) for +: 'PosixPath' and 'str'

A trivial fix is converting the specified filename to str when assigning to self._filename:

    -        self._filename = self._files[0]
    +        self._filename = str(self._files[0])

New changeset 06de1aeff94e524bed21d188065c4cd1590fb046 by ericvsmith (Zhiming Wang) in branch 'master':
bpo-31281: Fix pathlib.Path incompatibility in fileinput (gh-3208)
https://github.com/python/cpython/commit/06de1aeff94e524bed21d188065c4cd1590fb046

I did not backport this to 3.6, because it depends on other changes that themselves have not been backported.