BUG: Series creation with datetime64 with non-ns unit as object dtype

[sinhrks]

• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

found a bug related to datetime64 with non-ns unit.

# OK
pd.Series(np.array([np.datetime64('2011-01-01')]))
#0   2011-01-01
# dtype: datetime64[ns]

# OK
pd.Series([np.datetime64('2011-01-01')], dtype=object)
#0    2011-01-01
# dtype: object

# NG
pd.Series(np.array([np.datetime64('2011-01-01')]), dtype=object)
#0    1970-01-01 00:00:00.000014
# dtype: object

# OK
pd.Series(np.array([np.datetime64('2011-01-01', 'ns')]), dtype=object)
#0    2011-01-01 00:00:00
# dtype: object

[sinhrks]

Unrelated to the fix itself, but adding more tests to duplicated (this test revealed the bug).

[codecov-io]

Current coverage is 85.23% (diff: 100%)

Merging #13876 into master will increase coverage by <.01%

@@             master     #13876   diff @@
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  Files           140        140          
  Lines         50456      50458     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits          43007      43010     +3   
+ Misses         7449       7448     -1   
  Partials          0          0          

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[jreback]

thanks @sinhrks