[python-package] declare the use of inline type hints

[jameslamb]

Contributes to #3756.
Contributes to #3867.

This PR proposes updating the Python package to declare that inline type annotations are used.

• adds trove classifier Typing :: Typed
• adds a py.typed file

Why add the Typing :: Typed trove classifier?

This classifier helps researchers and others summarizing PyPI's contents to identify packages that export type annotations.

See the discussion in pypi/warehouse#4348.

There are 5200+ packages on PyPI using this classifier: https://pypi.org/search/?c=Typing+%3A%3A+Typed.

Why add a py.typed file?

As described in PEP 561 (link), including an empty file named py.typed tells tools like mypy that a package's source code contains type annotations.

That doesn't matter for lightgbm itself, but it's helpful for anyone writing code that depends on lightgbm (e.g. optuna, lightgbm-ray, lots of other user code) and wanting to check that code with mypy.

Consider the following example. Given the following file...

"""script.py"""
import lightgbm as lgb

clf = lgb.LGBMClassifier(n_estimators="not-a-number")

Running

mypy ./script.py

if you have lightgbm < 4.0 installed from PyPI or from master, you'll see the following error.

test.py:1: error: Skipping analyzing "lightgbm": module is installed, but missing library stubs or py.typed marker
test.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

But on this branch, mypy is able to check that code 😁

script.py:5: error: Argument "n_estimators" to "LGBMClassifier" has incompatible type "str"; expected "int"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

[github-actions]

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