to_datetime returning numpy.datetime64
#### Code Sample, a copy-pastable example if possible

This code:
```python
>>> df = pd.DataFrame({'date': ['Aug2020', 'November 2020']})
>>> df['parsed'] = df['date'].apply(pd.to_datetime)
>>> end = df.loc[df['parsed'].idxmax()]
>>> end['parsed'].replace(day=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'numpy.datetime64' object has no attribute 'replace'

```
worked in Pandas 0.25.3, but raises since 1.0.0.

I think there might be an issue with unboxing values when there are mixed types in the dataframe:

```python
>>> df = pd.DataFrame({'date': ['Aug2020', 'November 2020']})
>>> new = (
...     df
...     .assign(
...         parsed=lambda x: x['date'].apply(pd.to_datetime),
...         parsed2 = lambda x: x['date'].apply(pd.to_datetime)
...     )
... )
>>> new['parsed'].iloc[0]
Timestamp('2020-08-01 00:00:00')
>>> new.iloc[0]['parsed']
numpy.datetime64('2020-08-01T00:00:00.000000000') # unboxed type
>>> new2 = new.drop(columns=['date'])
>>> new2['parsed'].iloc[0]
Timestamp('2020-08-01 00:00:00')
>>> new2.iloc[0]['parsed']
Timestamp('2020-08-01 00:00:00') # boxed type now that we've dropped the string column

```
#### Problem description

to_datetime can "sometimes" result in a np.datetime64 return type.

np.datetime64 is not a valid return type for to_datetime (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.to_datetime.html), it should always be a datetimelike.

#### Expected Output

As in previous versions of Pandas:

```python
>>> df = pd.DataFrame({'date': ['Aug2020', 'November 2020']})
>>> df['parsed'] = df['date'].apply(pd.to_datetime)
>>> end = df.loc[df['parsed'].idxmax()]
>>> end['parsed'].replace(day=2)
Timestamp('2020-11-02 00:00:00')
```

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

<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : None.None

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 41.2.0
Cython           : 0.29.14
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.11.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
