Allowing UT1 as a DeltaTime here raises a question.  Is a DeltaTime meant to be a physical interval of time in a particular frame, or a mathematical interval describing the difference in the JD representation?  For example:
```
dt_ut1 = Time('2000:002', scale='ut1') - Time('2000:001', scale='ut1')
```
So is `dt_ut1` an absolute physical time interval in that frame or is it mathematical (delta_JD = 1.0)?  If it's physical then we should disallow UT1 as a `DeltaTime` scale.  If mathematical then I don't see a reason to not include UTC as a scale since the math works fine, it's just a more extreme example of non-uniform time.