[ci] [python-package] fix mypy error about return_cvbooster in cv()

[jameslamb]

Contributes to #3867.

Fixes the following error from mypy.

python-package/lightgbm/engine.py:766: error: Incompatible types in assignment (expression has type "CVBooster", target has type "List[float]")  [assignment]

That error is raised because mypy (correctly) infers that the return value for cv() is a Dict[str, List[float]] here:

LightGBM/python-package/lightgbm/engine.py

Line 704 in f74875e

results = collections.defaultdict(list)

LightGBM/python-package/lightgbm/engine.py

Lines 746 to 748 in f74875e

	for _, key, mean, _, std in res:
	results[f'{key}-mean'].append(mean)
	results[f'{key}-stdv'].append(std)

But then it tries to add an item that is a CVBooster here:

LightGBM/python-package/lightgbm/engine.py

Lines 765 to 766 in f74875e

	if return_cvbooster:
	results['cvbooster'] = cvfolds

I think just #type: ignore-ing this error is preferable to other possible fixes like:

• changing the type of results to Dict[str, Any] (which would reduce mypy's ability to catch issues in the main update loop in cv()
• changing that return statement to something like return {**results, 'cvbooster': cvfolds}, which would add to the peak memory usage for running cv() by temporarily copying results

[github-actions]

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