BUG: DateOffset addition gives UserWarning: Discarding nonzero nanoseconds in conversion
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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

```python
import pandas as pd

print("Timedelta")
print(pd.Timestamp("2020-01-01 18:00:00.000000001") + pd.Timedelta(days=1))
print("Dateoffset")
print(pd.Timestamp("2020-01-01 18:00:00.000000001") + pd.DateOffset(days=1))
```

Output:

```
$ python bug.py
Timedelta
2020-01-02 18:00:00.000000001
Dateoffset
sys:1: UserWarning: Discarding nonzero nanoseconds in conversion
2020-01-02 18:00:00.000000001
```


#### Problem description

This program produces correct answers in both cases, but gives a warning about discarding nanoseconds when using DateOffset. I think this is because internally it is converted to a pydatetime somewhere. The warning does not occur with Timedelta, presumably because it's just adding the raw values.

#### Expected Output

I expect not to get a warning

```
Timedelta
2020-01-02 18:00:00.000000001
Dateoffset
2020-01-02 18:00:00.000000001
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.9.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-33-generic
Version          : #36-Ubuntu SMP Wed Aug 15 16:00:05 UTC 2018
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.utf8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 49.6.0.post20210108
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
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
