MNT add python version requirement

My development tools (uv) were complaining about the missing
`requires-python` field in `pyproject.toml`. This adds a dependency on
at least 3.11, which is what scipy and numpy currently demands.

Author: jolars

pyproject.toml

@@ -22,6 +22,14 @@ dependencies = [
 ]
 dynamic = ["version"]
 
+requires-python = ">=3.11"
+
+classifiers = [
+  "Programming Language :: Python :: 3 :: Only",
+  "Programming Language :: Python :: 3.11",
+  "Programming Language :: Python :: 3.12",
+  "Programming Language :: Python :: 3.13",
+]
 
 [tool.setuptools.dynamic]
 version = {attr = "skglm.__version__"}