Update dependency openai to v1.21.0

Author: renovate[bot]

Pipfile

@@ -33,7 +33,7 @@ slack-sdk = "==3.27.1"
 gitpython = "==3.1.43"
 pandas = "==2.2.2"
 matplotlib = "==3.8.4"
-openai = "==1.20.0"
+openai = "==1.21.0"
 "discord.py" = "==2.3.2"
 "misskey.py" = "==4.1.0"
 websockets = "==12.0"