[python-package] add more type hints in basic.py

[jameslamb]

Contributes to #3756.

Adds some type hints basic.py.

• proposes using "ctypes._Pointer' as a type hint for ctypes pointers
• adds type hints on _InnerPredictor.__create_sparse_native(), _InnerPredictor.__get_num_preds(), _InnerPredictor.__pred_for_csr(), _InnerPredictor.__pred_for_csc()
• switches to using keyword arguments (instead of positional ones) in calls to those functions, to avoid "accidentally passed data in the wrong order" types of bugs and make it easier for people reading the code to understand what's goin on
  • (extra important for methods like these where most of the arguments all can be small integers... it'd be easy to make a mistake and not catch it)

This improves mypy's ability to catch bugs, and makes it a bit easier to understand what the code is doing when reading it.

Notes for Reviewers

Back in #5446, we talked about how to type-annotate ctypes pointers. At the time @StrikerRUS recommended using ctypes._Pointer #5446 (comment) but I proposed just using Any, because I was afraid that the name _Pointer beginning with _ meant that it was consider an internal implementation detail that might change in future versions of ctypes.

This PR proposes adopting that suggestion to use ctypes._Pointer. Since then, I've learned more about how mypy evaluates type annotations.

Specifically, per the mypy docs (link)

Type comments can’t cause runtime errors because comments are not evaluated by Python.

In a similar way, using string literal types sidesteps the problem of annotations that would cause runtime errors.

I think that using a string-literal type hint means that if a future ctypes version removes ctypes._Pointer, it'll break type hinting but not actually cause runtime issues for users. Given that, I now support using the more specific type annotation to improve mypy's ability to catch bugs.

Consider the following example:

cat > ./test.py <<EOL
import ctypes

def do_stuff(x: "ctypes._does_not_exist") -> None:
    print(x)

do_stuff("--- hello ---")
EOL

mypy fails with an informative error.

mypy ./test.py

test.py:3: error: Name "ctypes._does_not_exist" is not defined [name-defined]
Found 1 error in 1 file (checked 1 source file)

But the code runs without issue.

python ./test.py

--- hello ---

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