NFD

Python 2 reached its EOL on Jan 1, 2020

macOS 10.15 deprecated python and python2, urging users to switch to python3 (3.7.3 at the moment). It is very likely that macOS 10.16 will not include python 2 anymore. Homebrew does not offer a python 2 formula anymore.
Ubuntu 18.04 comes with python 3.6 only in the default install. 2.7 is still available in the official repo, but it's an additional package that we're requiring users to install.
CentOS 8 similarly does not provide python 2 by default, and goes a step further by not providing a /usr/bin/python symlink even if python 2 is installed by the user. Other RHEL-derived distros are presumably following the same approach.
Fedora uses python 3 as default since version 23, and has recently switched /usr/bin/python to point to python 3 with the Fedora 31 release.
Debian is working on removing python 2 support.
On Arch Linux, python 3 has been the default for several years.
Gentoo has a configurable /usr/bin/python symlink, pointing to python 3 by default.

It's time to switch our build system (waf) to require python 3. Initially the minimum required version should be 3.5 in order to support Ubuntu 16.04; after #5087, the requirement can be raised to >=3.6.