feat(ci): add sight-cov utility to download coverage reports

A new CLI is available, `sight-cov.py`. Here is the synopsis:

```shell
usage: sight-cov.py [-h] [-d] [--html] repository merge-request

Download the code coverage report from the latest pipeline of a merge-request.

positional arguments:
  repository     Name of the repository: "sight", "night", or any private repository.
  merge-request  Identifier of the merge-request, same as the one present in its URL.

options:
  -h, --help     show this help message and exit
  -d, --diff     Compute the diff of the coverage with the dev branch.
  --html         Html output instead of text.
```

Here is the output of the utility:

```shell
# ~/dev/sight/sight-git/sight-cov.py -d sight 470 --html 
Downloading merge-request !470 artifacts for project sight...

Coverage report is available at:
file:///home/user/dev/sight/.coverage/sight%21470%23301939/coverage/index.html

Downloading dev branch artifacts for project sight

Diff coverage report is available at:
file:///home/user/dev/sight/.coverage/report.html
```