BUG: Series creation with datetime64 with non-ns unit as object dtype
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] 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
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13876?src=pr) is 85.23% (diff: 100%)

> Merging [#13876](https://codecov.io/gh/pydata/pandas/pull/13876?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

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

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d4f95fd...00244d0](https://codecov.io/gh/pydata/pandas/compare/d4f95fdebb74db5edb09c743b48c4b03e0940591...00244d0ea9d81016307a7a6b0d82de94dce34bcb?src=pr)
 thanks @sinhrks 
