update-super-project.py

[Boost-tasks](https://github.com/boostorg/boost-tasks) has a PHP script update-super-project which runs from the webserver every 15 minutes and commits boost submodule updates to the main superproject (boostorg/boost). It could be categorized as a github maintenance script.

I've written (and now recently re-reviewed) a python script that will accomplish the task. https://github.com/boostorg/boost-tasks/blob/php7/python/update-super-project.py Based on testing, it's working fine.

@mclow, if you'd like to test and review also, the best way is to fork all boost repositories into your own account. Even better, into another account which has no boostorg permissions. Create a separate github account. Here's a script which automates forking all repositories and submodules to prepare the test environment. https://github.com/boostorg/boost-tasks/blob/php7/python/forkrepos.sh With that done, it's actually pretty quick and easy to be able to run update-super-project.py.

The website may use [Celery - Distributed Task Queue](https://docs.celeryq.dev/en/stable/) to run cron tasks. However we don't have to put all eggs in one basket to start with. This could be regular cron job on a regular linux machine. And then after some time, if celery is working smoothly, possibly consider moving it there.