Fix ConnectionResetError not being raised when the transport is closed

fixes aio-libs#7172

aiohttp/http_writer.py

@@ -72,7 +72,12 @@ def _write(self, chunk: bytes) -> None:
         self.buffer_size += size
         self.output_size += size
 
-        if self._transport is None or self._transport.is_closing():
+        # self._protocol.transport will be None if connection is closed
+        #
+        # We cannot check if self._transport is None here because
+        # we hold a reference to it even after the connection is closed.
+        #
+        if self._protocol.transport is None or self._transport.is_closing():
             raise ConnectionResetError("Cannot write to closing transport")
         self._transport.write(chunk)