Drop support for running on Python 3.5

[JukkaL]

Python 3.5 has reached its end of life. However, Ubuntu 16.04 LTS still ships with
Python 3.5 and reaches its end of life in April 2021, and I'd like to keep supporting 3.5
until April (unless it becomes too difficult).

My motivation for waiting this long is that it's fairly common for organizations to leave
upgrades until the last minute.

[JelleZijlstra]

@JukkaL is there anything still blocking this? It would be good to do this sooner rather than later so contributors can stop worrying about Python 3.5 support (e.g. in #10219).

[JukkaL]

I'd like to make at least one more public release with Python 3.5 support. We should be able to drop it (soon) afterwards. However, in the next release we can mark Python 3.5 as deprecated and any new features will no longer need to support 3.5.

[hauntsaninja]

mypy no longer supports 3.5, as of #10706