[3.7] bpo-34391: Fix ftplib test for TLS 1.3 (GH-8787) (GH-8789)

Read from data socket to avoid "[SSL] shutdown while in init" exception
during shutdown of the dummy server.

Signed-off-by: Christian Heimes <christian@python.org>

<!-- issue-number: [bpo-34391](https://www.bugs.python.org/issue34391) -->
https://bugs.python.org/issue34391
<!-- /issue-number -->
(cherry picked from commit 1590c39336)


Co-authored-by: Christian Heimes <christian@python.org>

Lib/test/test_ftplib.py

@@ -880,18 +880,23 @@ class TestTLS_FTPClass(TestCase):
         # clear text
         with self.client.transfercmd('list') as sock:
             self.assertNotIsInstance(sock, ssl.SSLSocket)
+            self.assertEqual(sock.recv(1024), LIST_DATA.encode('ascii'))
         self.assertEqual(self.client.voidresp(), "226 transfer complete")
 
         # secured, after PROT P
         self.client.prot_p()
         with self.client.transfercmd('list') as sock:
             self.assertIsInstance(sock, ssl.SSLSocket)
+            # consume from SSL socket to finalize handshake and avoid
+            # "SSLError [SSL] shutdown while in init"
+            self.assertEqual(sock.recv(1024), LIST_DATA.encode('ascii'))
         self.assertEqual(self.client.voidresp(), "226 transfer complete")
 
         # PROT C is issued, the connection must be in cleartext again
         self.client.prot_c()
         with self.client.transfercmd('list') as sock:
             self.assertNotIsInstance(sock, ssl.SSLSocket)
+            self.assertEqual(sock.recv(1024), LIST_DATA.encode('ascii'))
         self.assertEqual(self.client.voidresp(), "226 transfer complete")
 
     def test_login(self):

Misc/NEWS.d/next/Tests/2018-08-16-18-48-47.bpo-34391.ouNfxC.rst

@@ -0,0 +1 @@
+Fix ftplib test for TLS 1.3 by reading from data socket.