Merge remote-tracking branch 'remotes/philmd-gitlab/tags/python-next-20200207' into staging

- Python 3 cleanups:
  . Remove text about Python 2 in qemu-deprecated (Thomas)
  . Remove shebang header (Paolo, Philippe)
  . scripts/checkpatch.pl now allows Python 3 interpreter (Philippe)
  . Explicit usage of Python 3 interpreter in scripts (Philippe)
  . Fix Python scripts permissions (Paolo, Philippe)
  . Drop 'from __future__ import print_function' (Paolo)
  . Specify minimum python requirements in ReadTheDocs configuration (Alex)
- Test UNIX/EXEC transports with migration (Oksana)
- Added extract_from_rpm helper, improved extract_from_deb (Liam)
- Allow to use other serial consoles than default one (Philippe)
- Various improvements in QEMUMonitorProtocol (Wainer)
- Fix kvm_available() on ppc64le (Wainer)

# gpg: Signature made Fri 07 Feb 2020 15:01:56 GMT
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/python-next-20200207: (46 commits)
  .readthedocs.yml: specify some minimum python requirements
  drop "from __future__ import print_function"
  make all Python scripts executable
  scripts/signrom: remove Python 2 support, add shebang
  tests/qemu-iotests/check: Only check for Python 3 interpreter
  scripts: Explicit usage of Python 3 (scripts without __main__)
  tests/qemu-iotests: Explicit usage of Python3 (scripts without __main__)
  tests/vm: Remove shebang header
  tests/acceptance: Remove shebang header
  scripts/tracetool: Remove shebang header
  scripts/minikconf: Explicit usage of Python 3
  scripts: Explicit usage of Python 3 (scripts with __main__)
  tests: Explicit usage of Python 3
  tests/qemu-iotests: Explicit usage of Python 3 (scripts with __main__)
  tests/qemu-iotests/check: Allow use of python3 interpreter
  scripts/checkpatch.pl: Only allow Python 3 interpreter
  tests/acceptance/migration: Default to -nodefaults
  tests/acceptance/migration: Add the 'migration' tag
  tests/acceptance/migration: Test EXEC transport when migrating
  tests/acceptance/migration: Test UNIX transport when migrating
  ...

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