[Windows] require an existing directory and suport junctions in ntpath.ismount()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-12-01.10:40:54.136>
labels = ['type-bug', '3.8', '3.9', '3.10', 'library', 'OS-windows']
title = '[Windows] require an existing directory and suport junctions in ntpath.ismount()'
updated_at = <Date 2021-03-28.02:09:36.146>
user = 'https://bugs.python.org/jainankur'

bugs.python.org fields:

activity = <Date 2021-03-28.02:09:36.146>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Windows']
creation = <Date 2019-12-01.10:40:54.136>
creator = 'jainankur'
dependencies = ['37609']
files = []
hgrepos = []
issue_num = 38948
keywords = []
message_count = 7.0
messages = ['357674', '357675', '357722', '357892', '357893', '357896', '389628']
nosy_count = 8.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'lazka', 'jainankur', 'Jonathan Hsu']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38948'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']