0.28.3

Enhancements:

• Renamed @kopf.on.this to @kopf.subhandler for readability. #618

Bugfixes:

• Fixed the resource discovery in case of parallel requests (improper locks & checks). #615

Internals:

• Fixed the usage example of kopf.execute() to be asynchronous. #618
• CI functional tests fixed for ResourceWarnings due to pykube-ng's session left-overs. #619 #608