shutdown trigger and lifecycle tasks

[devkral]

Changes:

• fix wrong stacklevel of the loguru wrapper
• add shutdown trigger type
• add lifecycle task pattern
• fix wrong Exception raised when maximal instances were surpassed
• remove completly bogus exception (wrong name and context)
• fix docs
• update tests
• bump python requirement to python 3.9
• python >= 3.9 cleanup
• fix instances calculation per scheduler not per executor
• don't ignore invalid kwargs for executors
• bump version

[devkral]

you may need to check the single commits. I applied the python 3.9 style changes of ruff.