#240, #294: Drop Python 3.5. Add Python 3.9 support

• Drop support for Python 3.5
  • Requires Python 3.6.8
  • Remove test job
• Update to DuniterPy v0.60.0 which drops Python 3.5 support
• #294 (closed): Update with Py 3.5 support dropped
  • aiohttp updated to v3.7.1
  • multidict updated to v5.0 which drops Py 3.5 support
• Add test-3.9 job

Close #240 (closed).