Update peewee to 3.9.0

[pyup-bot]

This PR updates peewee from 3.8.2 to 3.9.0.

Changelog

3.9.0

* Added new document describing how to [use peewee interactively](http://docs.peewee-orm.com/en/latest/peewee/interactive.html).
* Added convenience functions for generating model classes from a pre-existing
database, printing model definitions and printing CREATE TABLE sql for a
model. See the "use peewee interactively" section for details.
* Added a `__str__` implementation to all `Query` subclasses which converts the
query to a string and interpolates the parameters.
* Improvements to `sqlite_ext.JSONField` regarding the serialization of data,
as well as the addition of options to override the JSON serialization and
de-serialization functions.
* Added `index_type` parameter to `Field`
* Added `DatabaseProxy`, which allows one to use database-specific decorators
with an uninitialized `Proxy` object. See 1842 for discussion. Recommend
that you update any usage of `Proxy` for deferring database initialization to
use the new `DatabaseProxy` class instead.
* Added support for `INSERT ... ON CONFLICT` when the conflict target is a
partial index (e.g., contains a `WHERE` clause). The `OnConflict` and
`on_conflict()` APIs now take an additional `conflict_where` parameter to
represent the `WHERE` clause of the partial index in question. See 1860.
* Enhanced the `playhouse.kv` extension to use efficient upsert for *all*
database engines. Previously upsert was only supported for sqlite and mysql.
* Re-added the `orwhere()` query filtering method, which will append the given
expressions using `OR` instead of `AND`. See 391 for old discussion.
* Added some new examples to the ``examples/`` directory
* Added `select_from()` API for wrapping a query and selecting one or more
columns from the wrapped subquery. [Docs](http://docs.peewee-orm.com/en/latest/peewee/api.htmlSelectQuery.select_from).
* Added documentation on using [row values](http://docs.peewee-orm.com/en/latest/peewee/query_operators.htmlrow-values).
* Removed the (defunct) "speedups" C extension, which as of 3.8.2 only
contained a barely-faster function for quoting entities.

**Bugfixes**

* Fix bug in SQL generation when there was a subquery that used a common table
expressions.
* Enhanced `prefetch()` and fixed bug that could occur when mixing
self-referential foreign-keys and model aliases.
* MariaDB 10.3.3 introduces backwards-incompatible changes to the SQL used for
upsert. Peewee now introspects the MySQL server version at connection time to
ensure proper handling of version-specific features. See 1834 for details.
* Fixed bug where `TimestampField` would treat zero values as `None` when
reading from the database.

[View commits](https://github.com/coleifer/peewee/compare/3.8.2...3.9.0)

Links

• PyPI: https://pypi.org/project/peewee
• Changelog: https://pyup.io/changelogs/peewee/
• Repo: https://github.com/coleifer/peewee/

[coveralls]

Coverage increased (+0.07%) to 84.218% when pulling 9767760 on pyup-update-peewee-3.8.2-to-3.9.0 into ba12e46 on master.