Rust: concerning Python version for src/etc/char_private.py

src/etc/char_private.py is used to generate src/libcore/char_private.rs (which is checked in to the repository, so the script is to be run by a developer making a change to it, not the build system).

char_private.py requires Python 3.5+—it uses FileNotFoundError (introduced in Python 3) and subprocess.run (introduced in Python 3.5)—which is slightly surprising, given that the build dependencies section in the repository's README lists "python 2.7 (but not 3.x)".

It almost pains me to report this, because if at all possible, I'd rather _not_ edit the script: there is _enough_ Python 2 code in the world; if we are ever to free ourselves from the dead hand of the past, it is intendant on technical pioneers like us to _listen_ when told that the month of July 2010 called while we were out and _they want their programming language back_. Still, it is valuable to (first and foremost) document and (secondarily) be consistent about what we require.