We already have tests for python 3.5 and 3.6 but not 3.7.

Fixes #838.

.gitlab-ci.yml: Add tests for python 3.7

Closes #838

See merge request !1074

Chandan Singh authored Jan 15, 2019 and Javier Jardón committed Jan 15, 2019

Starting from `pytest` version 4.1.0, `Node.get_marker()` has been
removed, and hence our tests break when running with newer versions of
`pytest`. It was deprecated since a while back but it has recently been
removed completely. Use `get_closest_marker()` as a replacement that is
suggested in the changelog, and seems to work fine for our use case.

See https://github.com/pytest-dev/pytest/pull/4564 for more context on
the upstream issue.

One way of verifying this change is that this should fix the recently
added `tests-fedora-update-deps` job, that was failing before due to
this issue.

conftest.py: Don't use deprecated get_marker() function

See merge request !1073

Found during #833.

`git rev-list --boundary tag..HEAD` unfortunately gives boundaries
that are deeper than should when there is a merge commit between `tag`
and `HEAD`. The common ancestory of the two parents of the merge is
a boundary instead of the parent of the branch that is not traversed.

`--ancestry-path` fixes this issue by restricting `git` traversing
those branches.

`git rev-list --boundary HEAD..HEAD` does not return any boundary.
So in this case we need to manually tag the HEAD as a boundary.