bpo-33674: asyncio: Fix SSLProtocol race (GH-7175) (#7187)

Fix a race condition in SSLProtocol.connection_made() of
asyncio.sslproto: start immediately the handshake instead of using
call_soon(). Previously, data_received() could be called before the
handshake started, causing the handshake to hang or fail.
(cherry picked from commit be00a5583a2cb696335c527b921d1868266a42c6)

Co-authored-by: Victor Stinner <vstinner@redhat.com>
diff --git a/Lib/asyncio/sslproto.py b/Lib/asyncio/sslproto.py
index 2bfa45d..ab43e93 100644
--- a/Lib/asyncio/sslproto.py
+++ b/Lib/asyncio/sslproto.py
@@ -592,10 +592,10 @@
         # (b'', 1) is a special value in _process_write_backlog() to do
         # the SSL handshake
         self._write_backlog.append((b'', 1))
-        self._loop.call_soon(self._process_write_backlog)
         self._handshake_timeout_handle = \
             self._loop.call_later(self._ssl_handshake_timeout,
                                   self._check_handshake_timeout)
+        self._process_write_backlog()
 
     def _check_handshake_timeout(self):
         if self._in_handshake is True: