gh-126500: test_ssl: Don't stop ThreadedEchoServer on OSError in ConnectionHandler; rely on __exit__ (GH-126503)

If `read()` in the ConnectionHandler thread raises `OSError` (except `ConnectionError`),
the ConnectionHandler shuts down the entire ThreadedEchoServer,
preventing further connections.
It also does that for `EPROTOTYPE` in `wrap_conn`.

As far as I can see, this is done to avoid the server thread getting stuck,
forgotten, in its accept loop. However, since 2011 (5b95eb90a7)
the server is used as a context manager, and its `__exit__` does `stop()` and `join()`.
(I'm not sure if we *always* used `with` since that commit, but currently we do.)

Make sure that the context manager *is* used, and remove the `server.stop()`
calls from ConnectionHandler.

Lib/test/test_ssl.py

@@ -2299,7 +2299,6 @@ class ThreadedEchoServer(threading.Thread):
                 # See also http://erickt.github.io/blog/2014/11/19/adventures-in-debugging-a-potential-osx-kernel-bug/
                 if e.errno != errno.EPROTOTYPE and sys.platform != "darwin":
                     self.running = False
-                    self.server.stop()
                     self.close()
                 return False
             else:
@@ -2436,10 +2435,6 @@ class ThreadedEchoServer(threading.Thread):
                     self.close()
                     self.running = False
 
-                    # normally, we'd just stop here, but for the test
-                    # harness, we want to stop the server
-                    self.server.stop()
-
     def __init__(self, certificate=None, ssl_version=None,
                  certreqs=None, cacerts=None,
                  chatty=True, connectionchatty=False, starttls_server=False,
@@ -2473,21 +2468,33 @@ class ThreadedEchoServer(threading.Thread):
         self.conn_errors = []
         threading.Thread.__init__(self)
         self.daemon = True
+        self._in_context = False
 
     def __enter__(self):
+        if self._in_context:
+            raise ValueError('Re-entering ThreadedEchoServer context')
+        self._in_context = True
         self.start(threading.Event())
         self.flag.wait()
         return self
 
     def __exit__(self, *args):
+        assert self._in_context
+        self._in_context = False
         self.stop()
         self.join()
 
     def start(self, flag=None):
+        if not self._in_context:
+            raise ValueError(
+                'ThreadedEchoServer must be used as a context manager')
         self.flag = flag
         threading.Thread.start(self)
 
     def run(self):
+        if not self._in_context:
+            raise ValueError(
+                'ThreadedEchoServer must be used as a context manager')
         self.sock.settimeout(1.0)
         self.sock.listen(5)
         self.active = True