Schedule weekly test runs against unfrozen dependencies

[consideRatio]

With tests scheduled to run regularly against dependencies that aren't frozen, we could detect issues arising over time due to changes in dependencies or the various execution environments, such as the GitHub actions runners having certain python unrelated dependencies such as versions of geckodriver etc.

Below is an example of triggers for a github actions test workflow that could be used.

on:
  pull_request:
    paths-ignore:
      - "docs/**"
      - "contrib/**"
      - "**.md"
      - ".github/workflows/*"
      - "!.github/workflows/test.yaml"
  push:
    paths-ignore:
      - "docs/**"
      - "contrib/**"
      - "**.md"
      - ".github/workflows/*"
      - "!.github/workflows/test.yaml"
    branches-ignore:
      - "dependabot/**"
      - "pre-commit-ci-update-config"
  schedule:
    # At 05:00 on Monday - https://crontab.guru
    - cron: "0 5 * * 1"
  workflow_dispatch:

[consideRatio]

Acceptance test failures showing up in the main branch of jupyterhub/jupyter-server-proxy from time to time, and when they do I often wonder what caused it. Since this project helps setup various things that could break against different jupyterlab versions or similar as well over time, it would be great to be able to see if tests has started failing here or not over time.