Xoto3

- Safer long-polling default for the SQS poll utility.

- `versioned_diffed_update_item` more fully supports the use of the
`nicename` keyword argument, providing more specific Exceptions in
the case of the item not existing before the update.
- Improved typing for various query helpers in `xoto3.dynamodb.query`.

- FailedRecordExceptions now contain the entire exception for each
failed record. This is a very minor feature.
- Removed some unnecessarily verbose logging to put library users back
in control.

- `map_tree` now supports postorder transformations via keyword argument.

- Improved DynamoDB Item-related Exceptions for `GetItem`,
`put_but_raise_if_exists`, and `versioned_diffed_update_item`.

- Allow any characters for attribute names in `add_variables_to_expression`.
- We have a lot of snake_cased attribute names. We should be able to use this function with those.