Update databases/py-peewee to 2.8.1.

Bugs fixed
- #821 - issue warning if Cython is old
- #822 - better handling of MySQL connections point for advanced
  use-cases.
- #313 - support equality/inequality with generic foreign key
  queries, and ensure get_or_create works with GFKs.
- #834 - fixed Python3 incompatibilities in the PasswordField,
  thanks @mosquito.
- #836 - fix handling of last_insert_id() when using APSWDatabase.
- #845 - add connection hooks to APSWDatabase.
- #852 - check SQLite library version to avoid calls to missing
  APIs.
- #857 - allow database definition to be deferred when using the
  connection pool.
- #878 - formerly .limit(0) had no effect. Now adds LIMIT 0.
- #879 - implement a __hash__ method for Model
- #886 - fix count() for compound select queries.
- #895 - allow writing to the foreign_key_id descriptor to set the
  foreign key value.
- #893 - fix boolean logic bug in model_to_dict().
- #904 - fix side-effect in clean_prefetch_query, thanks to
  @p.kamayev
- #907 - package includes pskel now.
- #852 - fix sqlite version check in BerkeleyDB backend.
- #919 - add runtime check for sqlite3 library to match MySQL and
  Postgres. Thanks @M157q

New features
- Added a number of SQLite user-defined functions and aggregates.
- Use the DB-API2 Binary type for BlobField.
- Implemented the lucene scoring algorithm in the sqlite_ext
  Cython library.
- #825 - allow a custom base class for ModelOptions, providing an
  extension
- #830 - added SmallIntegerField type.
- #838 - allow using a custom descriptor class with
  ManyToManyField.
- #855 - merged change from @lez which included docs on using
  peewee with Pyramid.
- #858 - allow arguments to be passed on query-string when using
  the db_url module. Thanks @RealSalmon
- #862 - add support for truncate table, thanks @dev-zero for the
  sample code.
- Allow the related_name model Meta option to be a callable that
  accepts the foreign key field instance.


(fhajny)