Integrate meta-guf-distro/integrate/gitlab-ci/upgrade-to-python-3.9

Commit: https://gitlab.com/garz-fricke/yocto/layers/meta-guf-distro/-/commit/b03da121bd3d1729e96e1d632ed7de70b06c8e70

Integrate gitlab-ci/upgrade-to-python-3.9 and 3 more

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Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/669113ff5a25b7cfae84220d5e3beee1b3bc7fb1

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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Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/1ef9007ed938d83fd7ed788a7fd7f7d4acd11ec2

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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Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/79e3437dc264bd60253b2ca26397016df2606756

accept_merge_request: refactoring

• Combine two return statements into one.
• Switch if statement order to remove else branch.

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Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/a139ffb573adaeb54e3435dc3fd1276789f0a078

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The outdated YAML 1.1 specification allowed values like 'yes' or 'no' for boolean fields 1.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.