BUG: behaviour of skipna=None changed on master

[mathause]

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandas.
• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd
import numpy as np
pd.Series([1.7, 0.5, np.nan]).sum(skipna=None)

Issue Description

In pandas 1.3.4 the example returns 2.2 on nightly it returns nan.

This lead to a test failure in xarray (we use pandas to double check a result - so we get a wrong expected value & could fix this but I still think this should not change silently).

Expected Behavior

I think this should still return 2.2 or raise a deprecation warning or an error.

Installed Versions

pandas was installed from scipy-wheels-nightly:

python -m pip install -i https://pypi.anaconda.org/scipy-wheels-nightly/simple --no-deps --pre --upgrade --force-reinstall  pandas

INSTALLED VERSIONS

commit : 9018d32
python : 3.9.7.final.0
python-bits : 64
OS : Linux
OS-release : 5.11.0-38-generic
Version : #42~20.04.1-Ubuntu SMP Tue Sep 28 20:41:07 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.4.0.dev0+972.g9018d327de
numpy : 1.20.3
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3.1
setuptools : 58.2.0
Cython : None
pytest : 6.2.5
hypothesis : 6.24.0
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.2
IPython : 7.28.0
pandas_datareader: None
bs4 : 4.10.0
bottleneck : 1.3.2
fsspec : 2021.10.1
fastparquet : None
gcsfs : None
matplotlib : 3.4.3
numexpr : 2.7.3
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.7.1
sqlalchemy : None
tables : None
tabulate : None
xarray : 0.19.1.dev89+gfdabf3be
xlrd : None
xlwt : None
numba : 0.54.1

[jbrockmendel]

xref #43980

[mathause]

Thanks! Yes this silently changes the behavior of skipna=None. Feel free to close this issue if you think this does not warrant a deprecation cycle. I'll update the test in xarray.

[jreback]

actually we should validate bool value eg a None should raise as it's not a valid value