REF: merge datetime_to_datetime64 into array_to_datetime

[jbrockmendel]

• closes #xxxx (Replace xxxx with the Github issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[jreback]

looks fine, can you merge master.

cc @mroeschke

[mroeschke]

Looks like doctests are failing, otherwise LGTM

[jbrockmendel]

The failed doctest looks like the new behavior is just better than the old behavior.

1001     - Finally, mixing timezone-aware strings and :class:`datetime.datetime` always
1002       raises an error, even if the elements all have the same time offset.
1003 
1004     >>> from datetime import datetime, timezone, timedelta
1005     >>> d = datetime(2020, 1, 1, 18, tzinfo=timezone(-timedelta(hours=1)))
1006     >>> pd.to_datetime(["2020-01-01 17:00 -0100", d])
Expected:
    Traceback (most recent call last):
        ...
    ValueError: Tz-aware datetime.datetime cannot be converted to datetime64
                unless utc=True
Got:
    DatetimeIndex(['2020-01-01 17:00:00-01:00', '2020-01-01 18:00:00-01:00'], dtype='datetime64[ns, pytz.FixedOffset(-60)]', freq=None)

[mroeschke]

Yup agreed. Looks like this example could be removed from the docstring

[mroeschke]

Or rather, this example this doctest be corrected and added as a unit test

[mroeschke]

@jbrockmendel looks like this is still failing the doc tests

=================================== FAILURES ===================================
______________ [doctest] pandas.core.tools.datetimes.to_datetime _______________
1013 
1014     >>> pd.to_datetime(['2018-10-26 12:00 -0530', '2018-10-26 12:00 -0500'],
1015     ...                utc=True)
1016     DatetimeIndex(['2018-10-26 17:30:00+00:00', '2018-10-26 17:00:00+00:00'],
1017                   dtype='datetime64[ns, UTC]', freq=None)
1018 
1019     - Inputs can contain both naive and aware, string or datetime, the above
1020       rules still apply
1021 
1022     >>> pd.to_datetime(['2018-10-26 12:00', '2018-10-26 12:00 -0530',
UNEXPECTED EXCEPTION: NameError("name 'timezone' is not defined")
Traceback (most recent call last):
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.8/doctest.py", line 1336, in __run
    exec(compile(example.source, filename, "single",
  File "<doctest pandas.core.tools.datetimes.to_datetime[18]>", line 4, in <module>
NameError: name 'timezone' is not defined
/home/runner/work/pandas/pandas/pandas/core/tools/datetimes.py:1022: UnexpectedException