fix: setup.py python_requires stale at >=3.9.16, docs require 3.10

[PARINAYKAPOOR]

Fixes part of #1179.

The installation docs (docs/EN/source/getting_started/installation.rst) require Python 3.10, but setup.py's python_requires still said >=3.9.16. This lets pip install proceed on Python 3.9 and fail deep inside dependency resolution (e.g. flashinfer-python requiring >=3.10) instead of surfacing a clear "Python >=3.10 required" error immediately.

Note: the sgl-kernel==0.3.7.post1 version-pin issue also reported in #1179 appears to already be resolved on main (now sglang-kernel==0.4.2.post1), so this PR addresses the remaining root cause: the stale python_requires metadata.

Test plan

• Confirmed current docs state Python 3.10 as the requirement (both the general requirements section and the conda create -n lightllm python=3.10 example).
• Confirmed setup.py was out of sync (>=3.9.16) with that documented requirement.
• One-line change, no other logic affected.