Tpot Classifier

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## I am trying to run import TPOTClassifier as `from tpot import TPOTClassifier` but I get the following message, #ModuleNotFoundError: No module named 'sklearn.externals.joblib'

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## It was working well before I run the below code that crash the importing task.

`pip install --upgrade --no-deps --force-reinstall git+https://github.com/weixuanfu/tpot.git@scoring_api_bug`

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## Process to reproduce the issue

[ordered list the process to finding and recreating the issue, example below]

1. User creates TPOT instance
2. User calls TPOT `fit()` function with training data
3. TPOT crashes with a `KeyError` after 5 generations

## Expected result

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## Current result

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## Possible fix

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## `name of issue` screenshot

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