python-3.9: upgrade python-gitlab

We're upgrading python-gitlab again, since the two problems with the new
version have been fixed.

The source branch not being deleted after merge [1] has been worked
around on our side [2].

The missing retry_transient_errors argument in the API call are actually
intended behavior [3].

[1]: https://github.com/python-gitlab/python-gitlab/issues/1750
[2]: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/1ef9007ed938d83fd7ed788a7fd7f7d4acd11ec2
[3]: https://github.com/python-gitlab/python-gitlab/issues/1749#issuecomment-994617547