0.0.21

[pomponchik]

A major and significant update that changes most of the basic use cases:

• You no longer need to wrap Storage class fields in a Field. This happens automatically behind the scenes.
• Field now has a shorthand alias: F.
• The secret flag has been renamed to hide. In future releases, the secret flag will be reused for other purposes.
• A new classifier for PyPi was added.
• The internal implementation of data sources was changed. Now, all built-in data sources are internally implemented as plugins based on the pristan system. Externally, the system’s behavior did not change, but this makes it possible to move data sources into separate packages in future releases.