Incorrect Optional typing on PageInfo hasPreviousPage and hasNextPage

The typing of `PageInfo.hasPreviousPage` and `PageInfo.hasNextPage` appears to be incorrectly indicated as `Optional[bool]` [here](https://github.com/graphql-python/graphql-relay-py/blob/ac5f88848fcad631034c4e54eba0bb7970e8b993/src/graphql_relay/connection/connectiontypes.py#L15-L16).

Is there a reason for this, or can it be fixed?