[Python] Add temporal unit checking in NumPyDtypeUnifier

[HyukjinKwon]

Describe the bug, including details regarding any error messages, version, and platform.

arrow/python/pyarrow/src/arrow/python/inference.cc

Line 258 in de6eb89

// TODO: check that units are all the same

For example,

import pyarrow as pa
import numpy as np
pa.array([np.datetime64('2020-01-01', 's'), np.datetime64('2020-01-02', 'ms')])

fails with somewhat a confusing error message:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pyarrow/array.pxi", line 375, in pyarrow.lib.array
  File "pyarrow/array.pxi", line 46, in pyarrow.lib._sequence_to_array
  File "pyarrow/error.pxi", line 155, in pyarrow.lib.pyarrow_internal_check_status
  File "pyarrow/error.pxi", line 92, in pyarrow.lib.check_status
pyarrow.lib.ArrowNotImplementedError: Expected np.datetime64 but got: timestamp[ms]

Component(s)

Python