Break up websocket 2 (#17028)

* Break up websocket 2

* Lint+Test

* Lintttt

* Rename

homeassistant/components/websocket_api/const.py

@@ -1,4 +1,11 @@
 """Websocket constants."""
+import asyncio
+from concurrent import futures
+
+DOMAIN = 'websocket_api'
+URL = '/api/websocket'
+MAX_PENDING_MSG = 512
+
 ERR_ID_REUSE = 1
 ERR_INVALID_FORMAT = 2
 ERR_NOT_FOUND = 3
@@ -6,3 +13,8 @@ ERR_UNKNOWN_COMMAND = 4
 ERR_UNKNOWN_ERROR = 5
 
 TYPE_RESULT = 'result'
+
+# Define the possible errors that occur when connections are cancelled.
+# Originally, this was just asyncio.CancelledError, but issue #9546 showed
+# that futures.CancelledErrors can also occur in some situations.
+CANCELLATION_ERRORS = (asyncio.CancelledError, futures.CancelledError)