I think this is a consequence of PEP380 and its decision to finalize the subgenerator when the delegating generator is closed.
Consider this simple example without tempfile:

def yielder (fileobj):
    yield from fileobj

with open('some_test_file', 'w') as f:
    f.write('line one\nline two\nline three')

with open('some_test_file', 'r') as f:
    line = next(yielder(f))
    nline = next(f)

==>

Traceback (most recent call last):
  File "<pyshell#11>", line 3, in <module>
    nline = next(f)
ValueError: I/O operation on closed file.

I think test_csv does the file-closing operation on lines 626/627 when it creates the temporary csv.reader(fileobj).

    def test_read_dict_fieldnames_from_file(self):
        with TemporaryFile("w+") as fileobj:
            fileobj.write("f1,f2,f3\r\n1,2,abc\r\n")
            fileobj.seek(0)
            reader = csv.DictReader(fileobj,
                                    fieldnames=next(csv.reader(fileobj)))
            self.assertEqual(reader.fieldnames, ["f1", "f2", "f3"])
            self.assertEqual(next(reader), {"f1": '1', "f2": '2', "f3": 'abc'})