os.path.ismount does not accept bytes objects

[NikolayAleksandrovichPavlov]

BPO	32693
Nosy	@pfmoore, @tjguk, @zware, @zooba
Superseder	bpo-32556: support bytes paths in nt _getdiskusage, _getvolumepathname, and _getfinalpathname

^{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 2018-01-28.21:12:56.488>
created_at = <Date 2018-01-28.11:16:16.002>
labels = ['type-bug', '3.7', 'OS-windows']
title = 'os.path.ismount does not accept bytes objects'
updated_at = <Date 2018-01-28.21:12:56.487>
user = 'https://bugs.python.org/NikolayAleksandrovichPavlov'

bugs.python.org fields:

activity = <Date 2018-01-28.21:12:56.487>
actor = 'eryksun'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-28.21:12:56.488>
closer = 'eryksun'
components = ['Windows']
creation = <Date 2018-01-28.11:16:16.002>
creator = 'Nikolay Aleksandrovich Pavlov'
dependencies = []
files = []
hgrepos = []
issue_num = 32693
keywords = []
message_count = 1.0
messages = ['310936']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'steve.dower', 'Nikolay Aleksandrovich Pavlov']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '32556'
type = 'behavior'
url = 'https://bugs.python.org/issue32693'
versions = ['Python 3.6', 'Python 3.7']

[NikolayAleksandrovichPavlov]

According to the documentation it ought to accept any path-like object, but actually throws for bytes ones. The trace of logic is the following:

1. ismount() from Lib/ntpath.py is using os.fpath which may return either bytes or str:

   cpython/Lib/ntpath.py

   Line 263 in 255f7a2

   path = os.fspath(path)

   .
2. The output is passed to _getvolumepathname unchanged.
3. The clinic defines that function’s only argument as unicode.
4. If clinic was not there it would still require to change how path_wchar is defined below, PyUnicode… functions will not accept bytes object:

   cpython/Modules/posixmodule.c

   Line 3808 in 255f7a2

   path_wchar = PyUnicode_AsUnicodeAndSize(path, &buflen);

   .

Suggestion: change [Lib/ntpath.py](https://github.com/python/cpython/blob/main/Lib/ntpath.py) to use os.fsdecode in place of os.fspath.

This is a relay of issue powerline/powerline#1876. Initial reporter observed bug on Python 3.6 (patch unknown), I could see an issue in current master (all links above point to that).