add .isort.cfg (#378)

python/sglang/api.py

@@ -3,11 +3,7 @@
 import re
 from typing import Callable, List, Optional, Union
 
-from sglang.backend.anthropic import Anthropic
 from sglang.backend.base_backend import BaseBackend
-from sglang.backend.openai import OpenAI
-from sglang.backend.runtime_endpoint import RuntimeEndpoint
-from sglang.backend.vertexai import VertexAI
 from sglang.global_config import global_config
 from sglang.lang.ir import (
     SglExpr,