[to_datetime] TypeError: Argument 'date_string' has incorrect type (expected str, got numpy.str_)
#### Code Sample

```python
import pandas as pd
import numpy as np

value = np.str_('2019-02-04 10:18:46.297000+0000')
arr = [value]
s = pd.Series(arr)

pd.to_datetime(value)  # good
pd.to_datetime(s.iloc[0])  # good
pd.to_datetime(arr[0])  # good
```
```python
pd.to_datetime(arr)  # error
```

```python
pd.to_datetime(s)  # error
```

Traceback:

<details>

```python
TypeError                                 Traceback (most recent call last)
pandas/_libs/tslib.pyx in pandas._libs.tslib.array_to_datetime()

TypeError: Expected unicode, got numpy.str_

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
<ipython-input-72-bab25c808085> in <module>
----> 1 pd.to_datetime(s)  # error

~/.virtualenvs/lr/lib/python3.7/site-packages/pandas/core/tools/datetimes.py in to_datetime(arg, errors, dayfirst, yearfirst, utc, format, exact, unit, infer_datetime_format, origin, cache)
    726             result = arg.map(cache_array)
    727         else:
--> 728             values = convert_listlike(arg._values, format)
    729             result = arg._constructor(values, index=arg.index, name=arg.name)
    730     elif isinstance(arg, (ABCDataFrame, abc.MutableMapping)):

~/.virtualenvs/lr/lib/python3.7/site-packages/pandas/core/tools/datetimes.py in _convert_listlike_datetimes(arg, format, name, tz, unit, errors, infer_datetime_format, dayfirst, yearfirst, exact)
    445             errors=errors,
    446             require_iso8601=require_iso8601,
--> 447             allow_object=True,
    448         )
    449 

~/.virtualenvs/lr/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py in objects_to_datetime64ns(data, dayfirst, yearfirst, utc, errors, require_iso8601, allow_object)
   1850             dayfirst=dayfirst,
   1851             yearfirst=yearfirst,
-> 1852             require_iso8601=require_iso8601,
   1853         )
   1854     except ValueError as e:

pandas/_libs/tslib.pyx in pandas._libs.tslib.array_to_datetime()

pandas/_libs/tslib.pyx in pandas._libs.tslib.array_to_datetime()

pandas/_libs/tslib.pyx in pandas._libs.tslib.array_to_datetime_object()

TypeError: Argument 'date_string' has incorrect type (expected str, got numpy.str_)
```

</details>

#### Problem description

`to_datetime` raises a `TypeError: Argument 'date_string' has incorrect type (expected str, got numpy.str_)` when the arg is a list-like of numpy.str_ objects, but not when the arg is a single numpy.str_ object.

#### Expected Output

I expect that it works with numpy.str_ like it does with str.
`assert (pd.to_datetime(s) == pd.to_datetime(s.astype(str))).all()`

Or at least to be consistent between `pd.to_datetime(s)` and `pd.to_datetime(s.iloc[0])`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-40-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 20.0.2
setuptools       : 45.0.0
Cython           : 0.29.13
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.2
html5lib         : None
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.2.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.0.2
numexpr          : None
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : 0.13.0
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.2.17
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : None
numba            : None

</details>
