BUG: find_common_type([interval[f8], interval[i8]]) returns object

[jbrockmendel]

I would expect it to return interval[f8], analogous to what we'd get with find_common_type([f8, i8])

[jorisvandenbossche]

IntervalDtype doesn't have its own specialization of _get_common_dtype, so it falls back to the base class implementation, which only looks at exact dtype equality.

So this can be fixed by adding a custom _get_common_dtype, and getting the common type of the underlying left/right arrays (the subdtype).
Now, I am not sure if this would be "better" or expected behaviour or not (not enough actual experience with intervals to judge that)