RuntimeError: 'path' must be None or a list, not <class '_frozen_importlib_external._NamespacePath'>

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## Context of the issue

[provide more detailed introduction to the issue itself and why it is relevant]

[the remaining entries are only necessary if you are reporting a bug]

## 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

[describe what you currently experience from this process, and thereby explain the bug]

## Possible fix

[not necessary, but suggest fixes or reasons for the bug]

## `name of issue` screenshot

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