pin unstructured-client (#2392)
Replacement for #2311 since python 3.8 was dropped as a supported
version.
Unstructured-client added `api_key_auth` as a param to
`UnstructuredClient` in [version
0.9.0](https://github.com/Unstructured-IO/unstructured-python-client/commit/8c93115c926765efe95c4e2a4e6bdf739fce41ad).
This pins the version of `unstructured-client` so users do not receive
`TypeError: UnstructuredClient.__init__() got an unexpected keyword
argument 'api_key_auth'`
