`pathlib.Path.is_mount()` calls `Path(self.parent)` and therefore misbehaves in `Path` subclasses

[barneygale]

BPO	39897
Nosy	@pitrou, @barneygale
PRs	bpo-39897: Remove needless Path(self.parent) call, which makes is_mount() misbehave in Path subclasses #18839

^{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 2020-04-17.17:46:21.221>
created_at = <Date 2020-03-08.04:11:08.791>
labels = ['type-feature', 'library', '3.9']
title = '`pathlib.Path.is_mount()` calls `Path(self.parent)` and therefore misbehaves in `Path` subclasses'
updated_at = <Date 2020-04-17.17:46:21.220>
user = 'https://github.com/barneygale'

bugs.python.org fields:

activity = <Date 2020-04-17.17:46:21.220>
actor = 'pitrou'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-17.17:46:21.221>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2020-03-08.04:11:08.791>
creator = 'barneygale'
dependencies = []
files = []
hgrepos = []
issue_num = 39897
keywords = ['patch']
message_count = 2.0
messages = ['363633', '366671']
nosy_count = 2.0
nosy_names = ['pitrou', 'barneygale']
pr_nums = ['18839']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39897'
versions = ['Python 3.9']

[barneygale]

pathlib.Path.is_mount() calls Path(self.parent), which:

• Is needless, as self.parent is already a Path instance!
• Prevents effective subclassing, as self.parent may be a Path subclass with its own stat() implementation

[pitrou]

New changeset c746c4f by Barney Gale in branch 'master':
bpo-39897: Remove needless Path(self.parent) call, which makes is_mount() misbehave in Path subclasses. (GH-18839)
c746c4f