我如何计算像'?‘这样的特殊字符？对于我在Pandas中DataFrame中的每一列？

import pandas as pd

df = pd.DataFrame({'col1': ['?', '1', '2', '3'],
                   'col2': ['4', '5', '?', '6'],
                   'col3': ['7', '8', '9', '?']})

df.replace('?', pd.NA, inplace=True)

df['col1'] = pd.to_numeric(df['col1'])
df['col2'] = pd.to_numeric(df['col2'])
df['col3'] = pd.to_numeric(df['col3'])

sum_col1 = df['col1'].sum()
mean_col2 = df['col2'].mean()