BUG: TimedeltaArray.astype(duration[{unit}][pyarrow])

[lukemanley]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/v2.0.0.rst file if fixing a bug or adding a new feature.

Fixes the following:

import pandas as pd

arr = pd.array([pd.Timedelta(days=1)]) 
arr.astype("duration[ns][pyarrow]")     # <- raises

TypeError: Cannot convert ArrowDtype to numpy.dtype

[mroeschke]

Thanks @lukemanley