update gitlab-ci.yml to use centos7

triggered by an update to autopep8 which fails to work on slc6...

@clemenci: FYI

mentioned in merge request !432 (merged)

I have a couple more changes that couls go in this MR: see https://gitlab.cern.ch/clemenci/Gaudi/commits/mr-main-433

added 1 commit

• 549a50b0 - Fixed the version of clang-format

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Applied.

added 1 commit

• c9fb6678 - Do not install argparse, not needed on CentOS7 (Python 2.7)

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mentioned in merge request !435 (merged)

added 1 commit

• be604bd8 - apply python formatting rules

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approved this merge request

merged

mentioned in commit 6e7a51e4

mentioned in merge request !416 (merged)

changed milestone to %v30r0

assigned to @hegner

added build system backport to v29 labels

mentioned in commit 7797309a

mentioned in merge request !441 (merged)

mentioned in commit 3e0f0e67

removed ~3693 backport to v29 labels