Automatically build a list of committers

Description

We should always have available a list of people who can accept merge requests.

Set up a python script which queries the GitLab API to build such a list and paste it into contrib/COMMITTERS.rst

This MR addresses issues raised in !1415 (closed) and works on top of those changes.

This merge request, when approved, will close:

#1071 (closed)

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