feat: make Python 3.7 the minimum version

- Python 3 version of Debian 10 (current stable) is 3.7.
  https://packages.debian.org/buster/python3
- Python 3 version of Ubuntu 20.04 (current LTS) is 3.8.
  https://packages.ubuntu.com/focal/python3

De facto, we don't test anymore on Python 3.6 and we won't change
this. In consequence, let's officially drop support for 3.6, that
might already be broken and that we would probably not want to fix
if it was the case.
13 jobs for !220 with topic/default/py37 in 7 minutes and 35 seconds (queued for 34 minutes and 4 seconds)
detached
Status Job ID Name Coverage
  Lint
passed black #390445

00:03:51

failed build_doc #390628
allowed to fail

00:02:58

passed check-manifest #390444

00:01:29

passed flake8 #390443

00:03:03

failed build_doc #390446
allowed to fail

00:02:53

failed build_doc #390600
allowed to fail

00:01:43

 
  Tests
failed py3-misc #390449

00:00:35

failed py3-server #390447

00:00:39

failed py3-web #390448

00:00:05

passed test-cube-skeleton #390629

00:02:55

failed test-cube-skeleton #390450

00:02:59

failed test-cube-skeleton #390599

00:02:15

 
  Qa
skipped can-i-merge #390451
allowed to fail
 
Name Stage Failure
failed
py3-misc Tests There has been a runner system failure, please try again
Running with gitlab-runner hpd-0.4.0 (e7173c85861f)
on runner-gitlab-runner-5b57d5ccf5-25xmd xxtUc89W
Preparing the "kubernetes" executor
Using Kubernetes namespace: gitlab-managed-apps
Using Kubernetes executor with image registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11 ...
Preparing environment
Waiting for pod gitlab-managed-apps/runner-xxtuc89w-project-385-concurrent-13rnxch to be running, status is Pending
ERROR: Job failed (system failure): prepare environment: image pull failed: Back-off pulling image "registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11". Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information
failed
py3-web Tests There has been a runner system failure, please try again
Running with gitlab-runner hpd-0.4.0 (e7173c85861f)
on runner-gitlab-runner-5b57d5ccf5-25xmd xxtUc89W
Preparing the "kubernetes" executor
Using Kubernetes namespace: gitlab-managed-apps
Using Kubernetes executor with image registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11 ...
Preparing environment
Waiting for pod gitlab-managed-apps/runner-xxtuc89w-project-385-concurrent-126hzgr to be running, status is Pending
ERROR: Job failed (system failure): prepare environment: image pull failed: Back-off pulling image "registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11". Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information
failed
py3-server Tests There has been a runner system failure, please try again
Running with gitlab-runner hpd-0.4.0 (e7173c85861f)
on runner-gitlab-runner-5b57d5ccf5-25xmd xxtUc89W
Preparing the "kubernetes" executor
Using Kubernetes namespace: gitlab-managed-apps
Using Kubernetes executor with image registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11 ...
Preparing environment
Waiting for pod gitlab-managed-apps/runner-xxtuc89w-project-385-concurrent-11t55hh to be running, status is Pending
Waiting for pod gitlab-managed-apps/runner-xxtuc89w-project-385-concurrent-11t55hh to be running, status is Pending
ERROR: Job failed (system failure): prepare environment: image pull failed: Back-off pulling image "registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11". Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information
failed
build_doc Lint
AttributeError: 'iclassmethod' object has no attribute '__func__'
The full traceback has been saved in /tmp/sphinx-err-e38n1gxu.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
ERROR: InvocationError for command /builds/cubicweb/cubicweb/.tox/doc/bin/python -m sphinx -b html -d /builds/cubicweb/cubicweb/.tox/doc/tmp/doctrees . /builds/cubicweb/cubicweb/.tox/doc/tmp/html (exited with code 2)
___________________________________ summary ____________________________________
ERROR: doc: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1