AttributeError: Can't pickle local object 'Check.greater_than.<locals>._greater_than'

[hebrd]

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• [x] I have checked that this issue has not already been reported.
• [ ] I have confirmed this bug exists on the latest version of pandera.
• [ ] (optional) I have confirmed this bug exists on the master branch of pandera.

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Code Sample, a copy-pastable example

# Your code here
schema = pa.DataFrameSchema(
  columns={"close": pa.Column(float, checks=[pa.Check.gt(0.0), ])},
  checks=[pa.Check(lambda df: df['low'] < df['high'])]
)
with open("test.pkl", "wb") as f:
  pickle.dump(schema, f)

Expected behavior

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Desktop (please complete the following information):

• OS: MacOS
• Version: 0.12.0

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