[python-package] remove hard dependency on 'scikit-learn', fix minimal runtime dependencies (#5942)

python-package/README.rst

@@ -56,6 +56,15 @@ To install all dependencies needed to use ``pandas`` in LightGBM, append ``[pand
 
     pip install 'lightgbm[pandas]'
 
+Use LightGBM with scikit-learn
+******************************
+
+To install all dependencies needed to use ``scikit-learn`` in LightGBM, append ``[scikit-learn]``.
+
+.. code:: sh
+
+    pip install 'lightgbm[scikit-learn]'
+
 Build from Sources
 ******************
 

python-package/pyproject.toml

@@ -16,6 +16,10 @@ classifiers = [
     "Programming Language :: Python :: 3.10",
     "Topic :: Scientific/Engineering :: Artificial Intelligence"
 ]
+dependencies = [
+    "numpy",
+    "scipy"
+]
 description = "LightGBM Python Package"
 license = {file = "LICENSE"}
 maintainers = [
@@ -34,6 +38,9 @@ dask = [
 pandas = [
     "pandas>=0.24.0"
 ]
+scikit-learn = [
+    "scikit-learn!=0.22.0"
+]
 
 [project.urls]
 homepage = "https://github.com/microsoft/LightGBM"

python-package/setup.cfg

 [options]
 include_package_data = True
-install_requires =
-    numpy
-    scikit-learn!=0.22.0
-    scipy
 
 [options.packages.find]
 where = lightgbm