[python-package] replace uses of scipy.sparse.issparse() with isinstance()

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Fixes the following mypy errors.

python-package/lightgbm/basic.py:2753: error: Value of type "Union[str, Path, ndarray[Any, Any], Any, Any, Any, Sequence, List[Sequence], List[ndarray[Any, Any]]]" is not indexable  [index]
python-package/lightgbm/basic.py:2753: error: No overload variant of "__getitem__" of "list" matches argument type "Tuple[List[int], slice]"  [call-overload]
python-package/lightgbm/basic.py:2753: note: Possible overload variants:
python-package/lightgbm/basic.py:2753: note:     def __getitem__(self, SupportsIndex, /) -> Sequence
python-package/lightgbm/basic.py:2753: note:     def __getitem__(self, slice, /) -> List[Sequence]
python-package/lightgbm/basic.py:2753: note:     def __getitem__(self, SupportsIndex, /) -> ndarray[Any, Any]
python-package/lightgbm/basic.py:2753: note:     def __getitem__(self, slice, /) -> List[ndarray[Any, Any]]
python-package/lightgbm/basic.py:2753: error: Invalid index type "Tuple[List[int], slice]" for "Union[str, Path, ndarray[Any, Any], Any, Any, Any, Sequence, List[Sequence], List[ndarray[Any, Any]]]"; expected type "Union[SupportsIndex, slice]"  [index]
python-package/lightgbm/basic.py:2753: error: Invalid index type "Tuple[List[int], slice]" for "Union[str, Path, ndarray[Any, Any], Any, Any, Any, Sequence, List[Sequence], List[ndarray[Any, Any]]]"; expected type "Union[int, slice, List[int]]"  [index]
python-package/lightgbm/basic.py:2905: error: Item "str" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2905: error: Item "Path" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2905: error: Item "Sequence" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2905: error: Item "List[Sequence]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2905: error: Item "List[ndarray[Any, Any]]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2913: error: Item "str" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "getformat"  [union-attr]
python-package/lightgbm/basic.py:2913: error: Item "Path" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "getformat"  [union-attr]
python-package/lightgbm/basic.py:2913: error: Item "Sequence" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "getformat"  [union-attr]
python-package/lightgbm/basic.py:2913: error: Item "List[Sequence]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "getformat"  [union-attr]
python-package/lightgbm/basic.py:2913: error: Item "List[ndarray[Any, Any]]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "getformat"  [union-attr]
python-package/lightgbm/basic.py:2931: error: Item "str" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2931: error: Item "Path" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2931: error: Item "Sequence" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2931: error: Item "List[Sequence]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2931: error: Item "List[ndarray[Any, Any]]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2945: error: Item "str" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2945: error: Item "Path" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2945: error: Item "Sequence" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2945: error: Item "List[Sequence]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]
python-package/lightgbm/basic.py:2945: error: Item "List[ndarray[Any, Any]]" of "Union[str, Any, Path, Sequence, List[Sequence], List[ndarray[Any, Any]]]" has no attribute "toarray"  [union-attr]

Proposes replacing calls like this:

if scipy.sparse.issparse(data): ...

with this:

if isinstance(data, scipy.sparse.spmatrix): ...

This helps mypy to understand branches of logic specific to sparse matrices.

It also protects lightgbm users a bit from future changes to scipy's public API.

We aren't getting much from scipy.sparse.issparse().

It's an alias for scipy.sparse.isspmatrix() (code link, docs link), which just runs isinstance(x, spmatrix) (code link).

There is also a recent, ongoing discussion in scipy about possibly changing how these functions handle sparse array (non-matrix) types: scipy/scipy#15790.

[github-actions]

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