databases/py-peewee: Update to 3.1.5.

## 3.1.5

- Fixed Python 2/3 incompatibility with `itertools.izip_longest()`.

## 3.1.4

- Added `BigAutoField` to support 64-bit auto-incrementing primary
  keys.
- Use Peewee-compatible datetime serialization when exporting JSON
  from a `DataSet`. Previously the JSON export used ISO-8601 by default.
- Added `Database.batch_commit` helper to wrap iterators in chunked
  transactions.

## 3.1.3

- Fixed issue where scope-specific settings were being updated
  in-place instead of copied.
- Fixed bug where setting a `ForeignKeyField` did not add it to the
  model's "dirty" fields list.
- Use pre-fetched data when using `prefetch()` with `ManyToManyField`.
- Use `JSON` data-type for SQLite `JSONField` instances.
- Add a `json_contains` function for use with SQLite `json1`  extension.
- Various documentation updates and additions.


(fhajny)