Issue #15747: skip chflags UF_IMMUTABLE tests if EOPNOTSUPP is raised.

This is necessary for ZFS systems, which don't support UF_IMMUTABLE.
--
Kubilay Kocak (koobs) asked me on IRC to backport this fix to Python 2.7: done!
diff --git a/Misc/NEWS b/Misc/NEWS
index 3d49637..cc69557 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -530,6 +530,10 @@
 Tests
 -----
 
+- Issue #15747: ZFS always returns EOPNOTSUPP when attempting to set the
+  UF_IMMUTABLE flag (via either chflags or lchflags); refactor affected
+  tests in test_posix.py to account for this.
+
 - Issue #16549: Add tests for json.tools.  Initial patch by Berker Peksag
   and Serhiy Storchaka.