Lint Python notebooks with ruff. (#12677)

The new ruff version fixed the blocking bugs, and I was able to fairly
easily us to a passing state: ruff fixed some issues on its own, I fixed
a handful by hand, and I added a list of narrowly-targeted exclusions
for files that are currently failing ruff rules that we probably should
look into eventually.

I went pretty lenient on the docs / cookbooks rules, allowing dead code
and such things. Perhaps in the future we may want to tighten the rules
further, but this is already a good set of checks that found real issues
and will prevent them going forward.

cookbook/custom_agent_with_plugin_retrieval.ipynb

@@ -33,14 +33,14 @@
    "outputs": [],
    "source": [
     "from langchain.agents import (\n",
-    "    Tool,\n",
     "    AgentExecutor,\n",
     "    LLMSingleActionAgent,\n",
     "    AgentOutputParser,\n",
     ")\n",
     "from langchain.prompts import StringPromptTemplate\n",
-    "from langchain.llms import OpenAI\nfrom langchain.utilities import SerpAPIWrapper\nfrom langchain.chains import LLMChain\n",
-    "from typing import List, Union\n",
+    "from langchain.llms import OpenAI\n",
+    "from langchain.chains import LLMChain\n",
+    "from typing import Union\n",
     "from langchain.schema import AgentAction, AgentFinish\n",
     "from langchain.agents.agent_toolkits import NLAToolkit\n",
     "from langchain.tools.plugin import AIPlugin\n",