difflib pathlike support for {unified,context}_diff() {from,to}file

[isidentical]

BPO	39311
Nosy	@tim-one, @rhettinger, @isidentical

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-01-13.03:43:34.704>
created_at = <Date 2020-01-12.12:58:02.508>
labels = ['library', '3.9']
title = 'difflib pathlike support for {unified,context}_diff() {from,to}file'
updated_at = <Date 2020-01-13.03:43:34.695>
user = 'https://github.com/isidentical'

bugs.python.org fields:

activity = <Date 2020-01-13.03:43:34.695>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-13.03:43:34.704>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2020-01-12.12:58:02.508>
creator = 'BTaskaya'
dependencies = []
files = []
hgrepos = []
issue_num = 39311
keywords = []
message_count = 2.0
messages = ['359847', '359885']
nosy_count = 3.0
nosy_names = ['tim.peters', 'rhettinger', 'BTaskaya']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39311'
versions = ['Python 3.9']

[isidentical]

>>> tuple(difflib.context_diff(["abc"], ["bcd"], fromfile=Path("example.py")))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/difflib.py", line 1254, in context_diff
    _check_types(a, b, fromfile, tofile, fromfiledate, tofiledate, lineterm)
  File "/usr/local/lib/python3.9/difflib.py", line 1301, in _check_types
    raise TypeError('all arguments must be str, not: %r' % (arg,))
TypeError: all arguments must be str, not: PosixPath('example.py')

IMHO to and from file arguments should accept PathLike objects. If agreed I can prepare a patch.

[rhettinger]

Elsewhere users of Path objects must explicitly coerce to str. For example: json.dumps(str(Path("example.py"))). This makes good sense to me. We don't want to add an dependency on pathlib just to recognize Path objects.