gh-153037: Fix AttributeError when iterating over a non-readable ZstdFile

Make `ZstdFile.__next__` raise `io.UnsupportedOperation` on non-readable
files, consistent with the other read methods and compression modules.

Author: lkk7

Lib/compression/zstd/_zstdfile.py

@@ -246,6 +246,7 @@ def peek(self, size=-1):
         return self._buffer.peek(size)
 
     def __next__(self):
+        self._check_can_read()
         if ret := self._buffer.readline():
             return ret
         raise StopIteration

Lib/test/test_zstd.py

@@ -2373,6 +2373,8 @@ def read(self, size):
                 f.read(100)
             with self.assertRaises(io.UnsupportedOperation):
                 f.seek(100)
+            with self.assertRaises(io.UnsupportedOperation):
+                next(iter(f))
         self.assertEqual(f.closed, True)
         with self.assertRaises(ValueError):
             f.readable()

Misc/NEWS.d/next/Library/2026-07-04-20-28-51.gh-issue-153037.XUSp_g.rst

@@ -0,0 +1,3 @@
+Fix :class:`~compression.zstd.ZstdFile` raising :exc:`AttributeError`
+instead of :exc:`io.UnsupportedOperation` when iterating over a file that
+is not open for reading.