Integrate gitlab-ci/upgrade-to-python-3.9 and 3 more
gitlab-ci: upgrade python image to 3.9
Fix pylint errors introduced with new pylint version:
- unspecified-encoding: explicitly set encoding when opening a file
- assigning-non-slot: use "set_reference()" function instead of assigning a value to the "reference" attribute
- unused-variable: remove "e" variable on exceptions when not referenced
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accept_merge_request: explicitly remove source branch
A bug in python-gitlab > 2.7.0 1 brought this up: In the past we relied on the merge request's setting, whether the source branch should be deleted or not after a merge. Since we're using the API's merge function in a fully automated task, however, the branch should always be deleted. Add the according argument explicitly.
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accept_merge_request: refactoring
- Combine two return statements into one.
- Switch if statement order to remove else branch.
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gitlab-ci: update bool values to YAML 1.2 spec
The outdated YAML 1.1 specification allowed values like 'yes' or 'no' for boolean fields 1. The newer spec 1.2 allows only 'true' and 'false' 2, so we update them to be conform with it.