databases/py-peewee: Update to 3.1.1.

- Fixed bug when using Model.alias() when the model defined a
  particular database schema.
- Added SchemaManager.create_foreign_key API to simplify adding
  constraints when dealing with circular foreign-key relationships.
- Improved implementation of Migrator.add_foreign_key_constraint so
  that it can be used with Postgresql (in addition to MySQL).
- Added PickleField to the playhouse.fields module. Docs.
- Fixed bug in implementation of CompressedField when using Python 3.
- Added KeyValue API in playhouse.kv module. Docs.
- More test cases for joining on sub-selects or common table
  expressions.

(fhajny)