[3.8] bpo-38811: Check for presence of os.link method in pathlib. (GH-17170)

[miss-islington]

Fix also the Path.symplink() method implementation for the case when
symlinks are not supported.
(cherry picked from commit 111772f)

Co-authored-by: Toke Høiland-Jørgensen toke@toke.dk

https://bugs.python.org/issue38811

[the-knights-who-say-ni]

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[miss-islington]

@tohojo and @serhiy-storchaka: Status check is done, and it's a success ✅ .

[miss-islington]

@tohojo and @serhiy-storchaka: Status check is done, and it's a success ✅ .

[miss-islington]

@tohojo and @serhiy-storchaka: Status check is done, and it's a success ✅ .

[vstinner]

I reject this PR because the change in master broke a buildbot:
#17170 (comment)