pathlib: move 'resolve()' logic out of path flavour

[barneygale]

BPO	43757
Nosy	@eryksun, @zooba, @willingc, @miss-islington, @barneygale
PRs	#25264 #26088 #26090 #26099 #26270

^{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 2021-04-28.15:50:36.035>
created_at = <Date 2021-04-07.02:32:48.519>
labels = ['type-feature', 'library', '3.10']
title = "pathlib: move 'resolve()' logic out of path flavour"
updated_at = <Date 2021-05-21.11:55:03.340>
user = 'https://github.com/barneygale'

bugs.python.org fields:

activity = <Date 2021-05-21.11:55:03.340>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-28.15:50:36.035>
closer = 'steve.dower'
components = ['Library (Lib)']
creation = <Date 2021-04-07.02:32:48.519>
creator = 'barneygale'
dependencies = []
files = []
hgrepos = []
issue_num = 43757
keywords = ['patch']
message_count = 5.0
messages = ['390397', '391962', '392229', '393623', '394107']
nosy_count = 5.0
nosy_names = ['eryksun', 'steve.dower', 'willingc', 'miss-islington', 'barneygale']
pr_nums = ['25264', '26088', '26090', '26099', '26270']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43757'
versions = ['Python 3.10']

[barneygale]

Under-the-hood functionality in pathlib is divided between:

• The 'flavour', which implements path syntax (separators, casefolding, etc)
• The 'accessor', which accesses the local (file)system.

The '_WindowsFlavour/_PosixFlavour.resolve()' function is misplaced, as it requires OS calls such as os.getcwd(), os.readlink(), and nt._getfinalpathname(). While the implementation does differ across Windows and POSIX, it's still properly part of the accessor interface, and not the flavour interface.

In preparation for addressing bpo-24132 I'd like to get these interfaces really tidy. Once bpo-39899 is fixed, this will be the last remaining flavour method that does accessor-y things.

[zooba]

I'm happy with the PR as it stands now. Anyone else have an opinion?

[zooba]

New changeset baecfbd by Barney Gale in branch 'master':
bpo-43757: Make pathlib use os.path.realpath() to resolve symlinks in a path (GH-25264)
baecfbd

[willingc]

New changeset ea14a07 by Miss Islington (bot) in branch '3.10':
bpo-43757: Document os.path.realpath(strict=True) in 3.10 whatsnew. (GH-26090) (bpo-26099)
ea14a07

[miss-islington]

New changeset ee51c56 by Barney Gale in branch '3.10':
[3.10] bpo-38671: Add test that pathlib.Path.resolve() returns an absolute path. (GH-26184) (GH-26270)
ee51c56