Create irrelevant_after decorator

[cbeauchesne]

Helping the declaration for version that changes a behavior. The idea is to use the version number where something has changed, and proposing a proper function name, rather asking user to handle with comparison operators

Basically, this :

@irrelevant_after("[email protected]", reason="This behavior has been deprecated on version 1.2.3")

is a sugar syntax for

@irrelevant(context.library >= "[email protected]", reason="This behavior has been deprecated on version 1.2.3")

Though, name could (should) be more explicit. What about :

@legacy_behavior(deprecated_at="[email protected]", reason="This behavior has been deprecated on version 1.2.3")