Fix httpx dependency version to be compatible with langchain

[ytkimirti]

> poetry add upstash_vector --optional
Using version ^0.2.0 for upstash-vector

Updating dependencies
Resolving dependencies... (0.2s)

Because upstash-vector (0.2.0) depends on httpx (>=0.25.0,<0.26.0)
 and no versions of upstash-vector match >0.2.0,<0.3.0, upstash-vector (>=0.2.0,<0.3.0) requires httpx (>=0.25.0,<0.26.0).
So, because langchain-community depends on both httpx (^0.24.1) and upstash-vector (^0.2.0), version solving failed.