[POC] Script to monitor all ETS package cron job

[kitchoi]

Background: Currently, inspection of the cron jobs requires someone to go to Travis.com, type in the name of the ETS package they want to check, look for the build that has the cron-job symbol on, and then loop over the ETS packages we have. This gets more and more laborious as the number of repositories grow.

It is possible to run a script to gather and present the status of the CI cron jobs for all the ETS packages, in one place.

This PR provides a proof-of-concept for future consideration. It provides a script that obtains the cron job results on GitHub Actions using its web API (Travis provides a web API so we can do the same thing), and then writes a file presenting the report.

To run, follow the instruction in check-cron/README.rst. The check-cron/result.md file is the file written by the script.

It should be relatively easy to create a GitHub Actions workflow just to run the script and update the file on the repo. Then we can access the result with a single URL.

This PR is open for the purpose of demonstrating an idea and persisting work towards it. It can be closed without merging.

(Note: It is not necessary for this script to be on master in order to run it on GitHub Actions.)