REGR: DataFrame.to_numpy(dtype=str) raises RuntimeError in pandas 1.1.0
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- [x ] I have confirmed this bug exists on the latest version of pandas.

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---

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#### Code Sample, a copy-pastable example

```python
import pandas as pd

df = pd.DataFrame(data=[[pd.Timestamp('2011-07-03 00:00:00'), 6628480.0]])
print(df.to_numpy(dtype=str))
```

#### Problem description

I try to convert the DataFrame to a Numpy array with dtype equal to str and I got the next error.

```python-traceback
Traceback (most recent call last):
  File "/main.py", line 4, in <module>
    df.to_numpy(dtype=str)
  File "/venv/lib/python3.7/site-packages/pandas/core/frame.py", line 1371, in to_numpy
    transpose=self._AXIS_REVERSED, dtype=dtype, copy=copy, na_value=na_value
  File "/venv/lib/python3.7/site-packages/pandas/core/internals/managers.py", line 826, in as_array
    arr = self._interleave(dtype=dtype, na_value=na_value)
  File "/venv/lib/python3.7/site-packages/pandas/core/internals/managers.py", line 864, in _interleave
    result[rl.indexer] = arr
RuntimeError: The string provided for NumPy ISO datetime formatting was too short, with length 1
```

#### Expected Output
```python
[['2011-07-03 00:00:00' '6628480.0']]
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.104-microsoft-standard
Version          : #1 SMP Wed Feb 19 06:37:35 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
