[test][python] fix pandas DeprecationWarning about is_sparse function

[StrikerRUS]

Fix the following error:

DeprecationWarning: is_sparse is deprecated and will be removed in a future version. Check `isinstance(dtype, pd.SparseDtype)` instead.

LightGBM requires pandas>=0.24.0

LightGBM/python-package/pyproject.toml

Lines 44 to 46 in 6c76b6b

	pandas = [
	"pandas>=0.24.0"
	]

So I checked that pd.SparseDtype is available in that version:
https://pandas.pydata.org/pandas-docs/version/0.24/reference/api/pandas.SparseDtype.html

Also, they've been performing isinstance check under the hood since the begging:
https://github.com/pandas-dev/pandas/blob/83eb2428ceb6257042173582f3f436c2c887aa69/pandas/core/dtypes/common.py#L216-L219

[jameslamb]

Awesome, thanks!

And thank you for the research. We do now also have CI jobs testing against the oldest versions of Python dependencies:

LightGBM/.github/workflows/python_package.yml

Line 120 in 4292b16

test-oldest-versions:

and the latest (even pre-released) versions:

LightGBM/.github/workflows/python_package.yml

Line 93 in 4292b16

test-latest-versions:

[StrikerRUS]

We do now also have CI jobs testing against the oldest versions of Python dependencies

Thank you for letting me know! That's great!

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