Fix EncodedFile.writelines

This is implemented by the underlying stream already, which additionally
checks if the stream is not closed, and calls `write` per line.

Ref/via: https://github.com/pytest-dev/pytest/pull/6558#issuecomment-578210807

testing/test_capture.py

@@ -1497,3 +1497,15 @@ def test_typeerror_encodedfile_write(testdir):
 def test_stderr_write_returns_len(capsys):
     """Write on Encoded files, namely captured stderr, should return number of characters written."""
     assert sys.stderr.write("Foo") == 3
+
+
+def test_encodedfile_writelines(tmpfile) -> None:
+    ef = capture.EncodedFile(tmpfile, "utf-8")
+    with pytest.raises(AttributeError):
+        ef.writelines([b"line1", b"line2"])  # type: ignore[list-item]  # noqa: F821
+    assert ef.writelines(["line1", "line2"]) is None  # type: ignore[func-returns-value]  # noqa: F821
+    tmpfile.seek(0)
+    assert tmpfile.read() == b"line1line2"
+    tmpfile.close()
+    with pytest.raises(ValueError):
+        ef.read()