bump websockets upper bound #616

looks like gql currently upper-bounds websockets to <16:

websockets==16.0 was released in Jan this year, and the latest version is 17.1. The breaking changes for 16 are just dropping support for old Python versions, which seems harmless. 17.0 has a longer list of changes that could require adaptation, though I haven't looked into how gql uses the library. (I'm just here because this is a transitive dependency for me, and another transitive dependency set websockets >= 16...)

FWIW I considered making a PR, but these tests fail on master for me, even before I've made any change:

FAILED tests/test_cli.py::test_cli_get_transport_appsync_websockets_iam[wss://XXXXXX.appsync-api.eu-west-3.amazonaws.com/graphql] - assert <gql.transport.appsync_websockets.AppSyncWebsocketsTransport object at 0x7a581e44a900> is None
FAILED tests/test_cli.py::test_cli_get_transport_appsync_websockets_iam[wss://noregion.amazonaws.com/graphql] - assert <gql.transport.appsync_websockets.AppSyncWebsocketsTransport object at 0x7a581e423ce0>
is None
FAILED tests/test_cli.py::test_cli_main_appsync_websockets_iam[wss://XXXXXX.appsync-api.eu-west-3.amazonaws.com/graphql] - gql.transport.exceptions.TransportConnectionFailed: Connect failed

(Based on a superficial examination I think this is possibly something to do with me having AWS credentials on this box)