Suggestion: Implement convenience methods from `NaiveWeek` on `IsoWeek`

[and-reas-se]

Hi,

NaiveWeek has the convenience methods first_day, last_day and days. But these are not implemented on IsoWeek. There is a previous issue #654 that proposed adding these to IsoWeek, but the PR (#666) that closed that issue as completed did not implement these methods on IsoWeek. Was this an oversight?

Either way, having these methods on IsoWeek as well would be useful, so I propose implementing them.

[djc]

Makes sense -- I'm happy to review a PR for these.