TST: MatplotlibDeprecationWarning in test related to pie chart
- [x] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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

```bash
pytest -Werror pandas/tests/plotting/tests_frame.py::TestDataFramePlots::test_pie_df_nan
```


#### Problem description

When testing ``test_frame.py`` the following warning is issued:
```
============================================================================================================= warnings summary =============================================================================================================
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_pie_df_nan
pandas\plotting\_matplotlib\core.py:1551: MatplotlibDeprecationWarning: normalize=None does not normalize if the sum is less than 1 but this behavior is deprecated since 3.3 until two minor releases later. After the deprecation period the default value will be normalize=True. To prevent normalization pass normalize=False
    results = ax.pie(y, labels=blabels, **kwds)

-- Docs: https://docs.pytest.org/en/stable/warnings.html
```

This code is the reason for the warnings:

```
matplotlib/axes/_axes.py
...
    def pie(...):
        ...
        if normalize is None:
            if sx < 1:
                cbook.warn_deprecated(
                    "3.3", message="normalize=None does not normalize "
                    "if the sum is less than 1 but this behavior "
                    "is deprecated since %(since)s until %(removal)s. "
                    "After the deprecation "
                    "period the default value will be normalize=True. "
                    "To prevent normalization pass normalize=False ")
            else:
                normalize = True
```

#### Expected Output

No warnings.

#### Versions

The issue is present starting from matplotlib version 3.3.
Before that ``normalize`` kwarg was not applicable to ``plt.pie``.

<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : 210c06e2f29bdb968fa715756bef9dcf1dfdc818
python           : 3.8.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1251

pandas           : 1.2.0.dev0+1079.g210c06e2f
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 49.6.0.post20201009
Cython           : 0.29.21
pytest           : 6.1.1
hypothesis       : 5.37.4
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.6.1
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 0.8.4
fastparquet      : 0.4.1
gcsfs            : 0.7.1
matplotlib       : 3.3.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.5.0
sqlalchemy       : 1.3.20
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.16.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.51.2
</details>