Change next release from 2.1 to 3.0

[MetRonnie]

I propose changing the upcoming release from 2.1 to 3.0 due to the breaking changes:

• Immutability means you can no longer do things like my_timepoint.year += 1
• Replacement of my_timepoint.get() with properties
• Improve TimeRecurrence class #183 is sort of like a breaking change

I don't see any reason not to make this a major release. Considering we have been bitten by packages not obeying semver recently (e.g. markupsafe), I think it would be more responsible to make this a major release (even if few people use isodatetime outside of Cylc)

If this is merged I will update the milestone accordingly

[oliver-sanders]

A little dramatic for the scale of changes but ok.