chore(requirements.txt): update dependencies to latest versions

This commit updates the dependencies in requirements.txt to their latest versions. Specifically, openai is updated to version 0.27.8, langchain to version 0.0.196, pypdf to version 3.9.1, chromadb to version 0.3.26, and tiktoken to version 0.4.0.

requirements.txt

@@ -1,6 +1,5 @@
-openai
-langchain
-pypdf
-chromadb
-tiktoken
-PyPDF2
+openai==0.27.8
+langchain==0.0.196
+pypdf==3.9.1
+chromadb==0.3.26
+tiktoken==0.4.0