Added requirements in preparation for Threema E2E support

[caronc]

Description

Related issue (if applicable): #apprise/1570

New updates to the Threema Plugin allows for E2E Encryption Support

Checklist

• Documentation ticket created (if applicable): apprise-docs/30
• The change is tested and works locally.
• No commented-out code in this PR.
• No lint errors (use tox -e lint and optionally tox -e format).
• Test coverage added or updated (use tox -e qa).

Testing

Anyone can help test as follows:

# Clone the branch
git clone -b threema-requirements https://github.com/caronc/apprise-api.git
cd apprise-api

# Run the unit tests
tox -e test

# Alternatively, spin up the full stack with Docker
docker compose up