_WriteResult subclasses redefine "__acknowledged" slot

The following classes redefine the __acknowledged slot from the _WriteResult base:

BulkWriteResult
DeleteResult
InsertManyResult
InsertOneResult
UpdateResult

This wastes a bit of memory on each instance.

The Python docs say:

If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.

Solution is to remove the duplicate slot. If you agree, I'd like to submit a PR to fix this.

I see there have been several _slots_  issues in pymongo in the past. I discovered this slot issue with slotscheck, a tool I maintain. If you like, I can add it to CI as I've done for instagram/LibCST, sqlalchemy/sqlalchemy, and aio-libs/aiohttp.