[ci] [python-package] fix mypy errors in Booster.refit()

[jameslamb]

Contributes to #3867.

Fixes the following errors from mypy.

python-package/lightgbm/basic.py:4198: error: Item "List[Any]" of "Union[ndarray[Any, Any], Any, List[Any]]" has no attribute "shape"  [union-attr]
python-package/lightgbm/basic.py:4228: error: Item "List[Any]" of "Union[ndarray[Any, Any], Any, List[Any]]" has no attribute "reshape"  [union-attr]

These errors come from the fact that mypy thinks it's possible for _InnerPredictor.predict() to return a List but in the lines those errors are reported on, the code uses the output of _InnerPredictor.predict() as numpy array unconditionally.

mypy doesn't know that when pred_leaf=True, the output of that function will never be a list.

[github-actions]

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