BUG: Read from S3 fails after assuming an AWS IAM role
- [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

assume_iam_role(role)
pd.read_csv(filepath)
```

#### Problem description

Hey all, I am having an issue where after assuming an AWS IAM role, I still get a forbidden error when running `pd.read_csv`. This error only happens in a Jupyter notebook. When just running a Python file, everything works (maybe related to the notebook kernel?).

Additionally, if I re-import Pandas after I assume the IAM role, then I am able to read from AWS without error.

Unfortunately, I have to assume my IAM role through a specific command that is internal to my company, so I am unable to share exactly how that part is happening (though I think it is mostly just calling the AWS CLI).

Could someone help me understand why this is happening and/or suggest a better way to handle this issue rather than re-importing pandas? I assume something is happening in the `__init__.py`, but after looking through this repo, I am unsure how the read from S3 stuff is being handled at all.

#### Expected Output

`pd.read_csv` works without error after assuming an IAM role.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.7.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.209-160.339.amzn2.x86_64
Version          : #1 SMP Wed Dec 16 22:44:04 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.1
numpy            : 1.19.5
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.3.3.post20210118
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : 3.4.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.5
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : 1.0.1
pyxlsb           : None
s3fs             : 0.5.2
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
