py-peewee: updated to 3.15.4

3.15.4

* Raise an exception in `ReconnectMixin` if connection is lost while inside a
  transaction (if the transaction was interrupted presumably some changes were
  lost and explicit intervention is needed).
* Add `db.Model` property to reduce boilerplate.
* Add support for running `prefetch()` queries with joins instead of subqueries
  (this helps overcome a MySQL limitation about applying LIMITs to a subquery).
* Add SQL `AVG` to whitelist to avoid coercing by default.
* Allow arbitrary keywords in metaclass constructor.
* Add a `pyproject.toml` to silence warnings from newer pips when `wheel`
  package is not available.

(adam)