Xoto3

Within `xoto3.dynamodb`:

- Fixed `batch_write` imports.
- `versioned_diffed_update_item` now performs standard
`boto3`-required data fixups on transformed items by default, but
this behavior can be customized.
- Fixed return type of `put_unless_exists`
- Fixed cases where reserved words used as table primary key attribute
names could cause `put_unless_exists` to fail with a
ValidationException.

- Added `require_index` utility to `dynamodb.query`
- New, clearer name `page` to replace `From` in `dynamodb.query`. The
previous name remains but is a deprecated alias.
- Made `dynamodb.batch_get.items_only` a bit more generic.

Fixed install_requires for Python > 3.6

Within `xoto3.dynamodb`:

- Fixed return value for `put_unless_exists`.
- Fixed type for `query.From`

Corrected naming in utility.

Initial release