re-add CancelledError exception handler case

This error is an Exception (not BaseException) in 3.7 and earlier.

Author: kristjanvalur

redis/asyncio/connection.py

@@ -705,6 +705,8 @@ async def connect(self):
             return
         try:
             await self._connect()
+        except asyncio.CancelledError:
+            raise  # in 3.7 and earlier, this is an Exception, not BaseException
         except (socket.timeout, asyncio.TimeoutError):
             raise TimeoutError("Timeout connecting to server")
         except OSError as e: