tqdm v4.53.0 stable

• provide get_new() helper for mixed subclasses in nested mode (#509)
• fix nested asyncio (#1074)
  • document async break hazard
• add tests
• drop py2.6/3.2/3.3 and distutils (no longer tested)
  • drop py2.6 (#502 <- #620, #127)
  • drop distutils in favour of setuptools/setup.cfg (#723, #721)
• CI framework overhaul
  • drop appveyor (Windows already tested by GHA)
  • skip devel PRs
  • automate linting comments on failure
• use setuptools_scm (#722)
  • fix & update tests
  • fix & upgrade snap build
  • update CONTRIBUTING docs