Release prep v0.6.0

[ircwaves]

Related Issue(s):

• Change validate to instance method #133

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[ircwaves]

FWIW, I think that the move to instance method here presents minimal impact to any existing implementations which upgrade to this release. @gadomski do you think this merits another Migration section like this:

0.5.x -> 0.6.0

Previously, the validate method was a classmethod, validating the payload
argument passed. This has now been made an instance method, which validates
the self._payload copy of the payload, from which the Task instance is
constructed. This is behaviorally the same, in that construction will fail if
validation fails, but allows implementers to utilize the instance method's
convenience functions.

Previous implementations of validate would have been similar to this:

    @classmethod
    def validate(payload: dict[str, Any]) -> bool:
        # Check The Things™
        return isinstance(payload, dict)

And will now need to be updated to this form:

    def validate(self) -> bool:
        # Check The Things™
        return isinstance(self._payload, dict)

[gadomski]

@gadomski do you think this merits another Migration section like this:

Can't hurt, right? :-)