add datetime64.__format__ similar to datetime.__format__

[gerritholl]

I would like for np.datetime64 to support the format-specification language using the __format__ method, so that it can be used in formatting strings. I frequently want to format plot labels using datetimes extracted from time coordinates in xarray.DataArray objects. Those have dtype M8[ns]. To format the date in a string directly, I need to convert these to datetime.datetime — due to the precision (see #12550) I need to manually cast to a lower time-resolution first. So rather than f'{d:%Y}' I need to write f'{d.astype("M8[s]").astype(datetime.datetime):%Y}, which is somewhat cumbersome.

Reproducing code example:

import numpy as np
d = np.datetime64(1234567890987654321, "ns")
print(f"{d:%Y}")

Error message:

Traceback (most recent call last):
  File "mwe84.py", line 3, in <module>
    print(f"{d:%Y}")
ValueError: Invalid format specifier

Numpy/Python version information:

Numpy 1.19.1, Python 3.8.5.

[eric-wieser]

Similar to #14243