Multiindexed columns (schemas) are not serialisable

[ConorForgie]

Pandas schemas for multi-indexed columns cannot be serialised to JSON

import pandas as pd
import pandera.pandas as pa


# Create a simple schema with MultiIndex column names (tuples)
schema = pa.DataFrameSchema({
    ('level1', 'col_a'): pa.Column(float),
    ('level1', 'col_b'): pa.Column(float),
    ('level2', 'col_c'): pa.Column(int),
})

# Create a matching DataFrame with MultiIndex columns
df = pd.DataFrame({
    ('level1', 'col_a'): [1.0, 2.0, 3.0],
    ('level1', 'col_b'): [4.0, 5.0, 6.0],
    ('level2', 'col_c'): [7, 8, 9],
})

print('DataFrame columns:')
print(df.columns)

# Validation works fine
validated = schema.validate(df)

# But serialization to JSON fails
json_str = schema.to_json()

Failes with

TypeError: keys must be str, int, float, bool or None, not tuple