in poplib, limit maximum line length that we read from the network (closes #16041)

Patch from Berker Peksag.
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index af48fdd..23d6887 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -198,6 +198,10 @@
                     113)
         self.assertEqual(self.client.retr('foo'), expected)
 
+    def test_too_long_lines(self):
+        self.assertRaises(poplib.error_proto, self.client._shortcmd,
+                          'echo +%s' % ((poplib._MAXLINE + 10) * 'a'))
+
     def test_dele(self):
         self.assertOK(self.client.dele('foo'))