Add initial gitlab-ci workflow file

[sjvrijn]

Description

• I have read the contribution guidelines
• This update is in line with what is recommended in the Python chapter of the Guide
• All user facing changes have been added to CHANGELOG.md

Adds a gitlab-ci.yml file to provide similar functionality as the current GitHub workflows:

• Automated tests on all supported Python versions, but only on Linux
• CFF convert validation job (should only run if CITATION.cff file is changed)
• Lychee link checker. I didn't get around to adding the cron-job running, or the issue creation based on link errors from it.

See this GitLab repo's CI output for output examples. Tests pass, CFF convert and links checker fail as expected for a default generated package.

Note changelog entry is still lumped in with 'unreleased' entries belonging to v0.5.0. See #455

Related issues:

• Consider adding gitlab support by the template #405
• Add gitlab CI workflow #452

Instructions to review the pull request

Create a python-template-test repo on GitLab (will be overwritten if existing)

# Create a temporary directory by running the following command. Keep the XXXXXX in the directory name. 
cd $(mktemp -d --tmpdir py-tmpl-XXXXXX)
# Use --vcs-ref <pr-branch> to point to the branch you want to test
copier copy --vcs-ref 452-gitlab-ci-workflow https://github.com/NLeSC/python-template .
# Fill with python-template-test info
# Create a local git repo to push to GitLab to trigger CI actions
git init
git add --all
git commit -m "First commit"
git remote add origin git@gitlab.com:<you>/python-template-test.git
git push -u origin main -f