BUG: ArrowDtype.numpy_dtype returning ns units for non-nano timestamp and duration types

[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/v2.1.0.rst file if fixing a bug or adding a new feature.

ArrowDtype.numpy_dtype goes through the pyarrow to_pandas_type method which was added before pandas supported non-nano units. This PR ensures the correct units are passed along to the numpy dtype. This fixes 6 xfails.

pyarrow already preserves the units when going directly from pyarrow to numpy:

>>> import pyarrow as pa
>>> arr = pa.array([1, 2, 3], type=pa.duration("us"))
>>> arr.to_numpy()
array([1, 2, 3], dtype='timedelta64[us]')

[jbrockmendel]

thanks @lukemanley