fix: Make flask an extra dependency

https://github.com/geekan/MetaGPT/pull/1118#discussion_r1540656272

requirements.txt

@@ -69,5 +69,4 @@ imap_tools==1.5.0  # Used by metagpt/tools/libs/email_login.py
 qianfan==0.3.2
 dashscope==1.14.1
 rank-bm25==0.2.2  # for tool recommendation
-jieba==0.42.1  # for tool recommendation
-flask   # for web api
+jieba==0.42.1  # for tool recommendation

setup.py

@@ -38,6 +38,7 @@ extras_require = {
         "llama-index-vector-stores-faiss==0.1.1",
         "chromadb==0.4.23",
     ],
+    "web-api": ["flask==3.0.2"],
 }
 
 extras_require["test"] = [