WARNING: The changes are backward-compatible (in theory). But the changes are also massive, so things can break unexpectedly (in practice). Test this upgrade carefully.

New features:

• @kopf.daemon for background resource-accompanying tasks/threads. #330 #342 #360
• @kopf.timer for regular and/or delayed activities & checks. #330 #342 #360
• kopf.PRESENT for labels/annotations filters (instead of misleading None). #327
• kopf.ABSENT for labels/annotations filters. #327
• Arbitrary callbacks for labels/annotations filters. #328
• kopf.all_(), kopf.any_(), kopf.not_(), kopf.none_() helpers for callbacks aggregation. #345
• A proper official documented way to configure operators; plus docs. #336 #337 #331
• Identities of operators, allowing multiple non-colliding operators for the same resources. #331 #344
• Configurable client-side timeouts for watch requests. #322 #336
• Configurable storages for handling state & diff-bases. #331 #346 #353

Improvements:

• Internal instant functions (lifecycles, when-filters) do not use thread pools anymore. #326
• Bodies & patches now have magical properties for metadata, spec, status, with "live view" into data. #327
• Labels and annotations are passed as additional kwargs (similar to body/spec/meta/status). #327
• All user-facing types and classes are published as framework's public interface (via top-level package). #341
• Switch KopfExample & KopfPeering from v1beta1 to v1 CRD API; keep v1beta1 nearby. #364
• Documentation is restructured for separate pages for all aspects of handling (kwargs, filters, etc). #325
• Explain the "structural schemas" of K8s 1.16+, and v1-vs-v1beta1 CRD differences. #364

Fixes:

• Threads from the sync handlers executors are not leaked (not left orphaned) at exit. #326
• Handlers' state is now persisted in annotations, not in status: for custom & built-in resources. This solves the issues with built-ins not handled properly in K8s 1.16+. #331
• Fix the on-field handler not being invoked when the diff is too generic (relative to the field). #340
• Provoke after-sleep handling cycles even on resources with no arbitrary field preservation. #343
• Fix premature finalizer removal with 2+ deletion handlers. #361
• Avoid no-effect patching and potentially delaying handling cycles. #362

Internal changes:

• Coveralls is made optional. #363
• Compatibility with Kubernetes 1.18 in CI/CD. #335
• pytest & pytest-asyncio are pinned temporarily until fixed on their side. #352