Deploy to Google App Engine Flex and ModuleNotFoundError for ‘streamlit_image_select’

I am trying to deploy a basic Streamlit app to Google App Engine Flex. Here are all the parts:

requirements.txt

streamlit
streamlit-image-select

app.yaml

runtime: python
env: flex

runtime_config:
  python_version: 3

handlers:
- url: /images
  static_dir: images

entrypoint: streamlit run app.py --server.port $PORT

app.py

import streamlit as st
from streamlit_image_select import image_select

CatPics = [
"https://i.ibb.co/Bt40nKL/cat1.jpg",
"https://i.ibb.co/qJG1x7w/cat2.jpg",
"https://i.ibb.co/9NJv0Cj/cat3.jpg"
]

Cat = image_select("Cats", CatPics)
st.write(Cat)

This app can be viewed in the browser when operated from localhost using python3 -m streamlit run app.py

The problem arises when I try to deploy to Google App Engine Flex. I attempt to deploy from my localhost using gcloud app deploy but receive an error saying Could not find a version that satisfies the requirement streamlit-image-select (from -r requirements.txt (line 2))

If I remove the streamlit-image-select line from the app.yaml file and then deploy again using gcloud app deploy, the build process completes without error, however, when I load the app in the browser it throws an error ModuleNotFoundError: No module named 'streamlit_image_select'

Where am I going wrong? What do I need to do so that I can get this app deployed to Google App Engine Flex ?