df.groupby(by=["b"], dropna=False).sum() returns"groupby() got an unexpected keyword argument 'dropna'"
Using the official documentation for groupby: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.groupby.html

the dropna=False argument does not work even with given example. 

l = [[1, 2, 3], [1, None, 4], [2, 1, 3], [1, 2, 2]]
df = pd.DataFrame(l, columns=["a", "b", "c"])
df.groupby(by=["b"], dropna=False).sum()