ci: use the python3.7 docker image which contains git

It is needed to be able to clone the pytest-capture-deprecatedwarnings
package.
2 jobs for topic/default/pytest_deprecated in 5 minutes and 10 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Tests
passed lint #58732

00:00:40

passed py3 #58733

00:05:10