Drop support for Python 3.8

[niklasf]

https://docs.python.org/3.9/whatsnew/3.9.html

• [x] Pidfd child watcher:
  • Documentation: https://docs.python.org/3.9/library/asyncio-policy.html#asyncio.PidfdChildWatcher
  • BPO: https://bugs.python.org/issue38692
  • Feature detection: https://github.com/python/cpython/blob/3ccdd9b180f9a3f29c8ddc8ad1b331fe8df26519/Lib/test/test_asyncio/test_subprocess.py#L695-L702 & https://github.com/vstinner/cpython/blob/fcd4a18a4b80f857d9855648ba69b7af58f46582/Lib/test/test_posix.py#L1473

As of 2021-07-13, readthedocs.org is limited to Python 3.8.

[niklasf]

• [x] Investigate start_new_session argument for popen.

[Xyrio]

fyi: 3.8 is last python supporting windows 7