bpo-39047: Skip chflags UF_IMMUTABLE tests if EOPNOTSUPP is raised. (GH-17607)

[attilajeges]

This is necessary for ZFS systems, which don't support UF_IMMUTABLE.

https://bugs.python.org/issue39047

[the-knights-who-say-ni]

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[serhiy-storchaka]

Thank you for your PR. Unfortunately this PR was unnoticed. The test was finally fixed 4 years later in other issue.