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/deeplake_semantic_search_over_chat.ipynb

@@ -50,15 +50,13 @@
    "source": [
     "import os\n",
     "import getpass\n",
-    "from langchain.document_loaders import PyPDFLoader, TextLoader\n",
     "from langchain.embeddings.openai import OpenAIEmbeddings\n",
     "from langchain.text_splitter import (\n",
     "    RecursiveCharacterTextSplitter,\n",
     "    CharacterTextSplitter,\n",
     ")\n",
     "from langchain.vectorstores import DeepLake\n",
-    "from langchain.chains import ConversationalRetrievalChain, RetrievalQA\n",
-    "from langchain.chat_models import ChatOpenAI\n",
+    "from langchain.chains import RetrievalQA\n",
     "from langchain.llms import OpenAI\n",
     "\n",
     "os.environ[\"OPENAI_API_KEY\"] = getpass.getpass(\"OpenAI API Key:\")\n",