Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-and-python-021020...

Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-and-python-021020-1' into staging

Python testing updates:

  - drop python 3.5 test from travis
  - replace Debian 9 containers with 10
  - increase cross build timeout
  - bump minimum python version in configure
  - move user plugins tests to gitlab
  - split deprecated builds into build and test

# gpg: Signature made Fri 02 Oct 2020 12:34:36 BST
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-testing-and-python-021020-1:
  gitlab: split deprecated job into build/check stages
  gitlab: move linux-user plugins test across to gitlab
  configure: Bump the minimum required Python version to 3.6
  gitlab-ci: Increase the timeout for the cross-compiler builds
  tests/docker: Remove old Debian 9 containers
  shippable.yml: Remove the Debian9-based MinGW cross-compiler tests
  tests/docker: Update the tricore container to debian 10
  gitlab-ci: Remove the Debian9-based containers and containers-layer3
  tests/docker: Use Fedora containers for MinGW cross-builds in the gitlab-CI
  travis.yml: Drop the Python 3.5 build
  travis.yml: Drop the superfluous Python 3.6 build
  travis.yml: Update Travis to use Bionic and Focal instead of Xenial
  travis.yml: Drop the default softmmu builds
  migration: Silence compiler warning in global_state_store_running()

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>