GHA: Exclude Python 3.9 runs for milestone branch

[felixfontein]

SUMMARY

Fixes GHA CI.

Ref: ansible-collections/news-for-maintainers#56

ISSUE TYPE

• Test Pull Request

COMPONENT NAME

GHA sanity and unit tests

[felixfontein]

Another reason why simply running ansible-test sanity --docker and ansible-test units --docker would be better (ansible-network/github_actions#120) - that automatically uses the supported Python versions for every ansible-core version.

[softwarefactory-project-zuul]

Build succeeded.
https://ansible.softwarefactory-project.io/zuul/buildset/ac9f84543ebe4aa6a6fd0b429c331d16

❌ ansible-galaxy-importer FAILURE in 5m 39s (non-voting)
✔️ build-ansible-collection SUCCESS in 14m 44s

[felixfontein]

The corresponding change was already made for ansible-network/github_actions in ansible-network/github_actions@ad77b41.

[softwarefactory-project-zuul]

Build succeeded (gate pipeline).
https://ansible.softwarefactory-project.io/zuul/buildset/7348f1ac6e3641a488ab05d735cac49e

❌ ansible-galaxy-importer FAILURE in 5m 08s (non-voting)
✔️ build-ansible-collection SUCCESS in 12m 29s

[softwarefactory-project-zuul]

Pull request merge failed: Resource not accessible by integration, You may need to manually rebase your PR and retry.

[felixfontein]

@alinabuzachis @tremble thanks!