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

changelog/6566.bugfix.rst

@@ -0,0 +1 @@
+Fix ``EncodedFile.writelines`` to call the underlying buffer's ``writelines`` method.

src/_pytest/capture.py

@@ -9,6 +9,7 @@ import os
 import sys
 from io import UnsupportedOperation
 from tempfile import TemporaryFile
+from typing import List
 
 import pytest
 from _pytest.compat import CaptureIO
@@ -426,9 +427,8 @@ class EncodedFile:
             )
         return self.buffer.write(obj)
 
-    def writelines(self, linelist):
-        data = "".join(linelist)
-        self.write(data)
+    def writelines(self, linelist: List[str]) -> None:
+        self.buffer.writelines([x.encode(self.encoding, "replace") for x in linelist])
 
     @property
     def name(self):

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()