Add support and set-up Python 3.9 test job

Python 3.9 have been released as stable on October 5th 2020

Tasks

• Create v3.9 container: docker/python3/poetry!1 (merged)
• Fix C(++) dependencies which do not provides wheels
  • aiohttp v3.7.x allows to use multidict 5.0.0 which has pre-built wheels for Py3.9, not the previous version. But, multidict v5.0 also drops Py 3.5 support. This has to be dropped from Silkaj too.
  • cffi used from pynacl
    • #176 (closed): Get rid of pynacl: pynacl is using an old version of cffi which is not built for py3.9. getting rid of pynacl from silkaj could allow us to remove libffi-dev. duniterpy#83 is planning to use pynacl.
  • typed-ast, a dependency of black is not pre-built for Py3.9. Now provided.
• Set-up tests-3.9 job