Support Python 3 to build

Currently qemu's configure requires Python 2 to build.  As Python 2 is rapidly approaching its EOL, it should be possible to build qemu with Python 3.

Python 2 EOL is in 2020 so there is a fair amount of time.

QEMU is transitioning to support both Python 2.6+ and 3 but most Python code has not been converted yet.

You are welcome to contribute patches:
https://wiki.qemu.org/index.php/Contribute/SubmitAPatch

> Python 2 EOL is in 2020 so there is a fair amount of time.

Not as much time as you might think. Even well before that time, any new releases of long life and/or enterprise distros are likely to choose to skip py2 by default, as it will be EOL long before the distro itself EOLs. 

Two patches series posted in Aug last year:

https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg03642.html
https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg06528.html

An updated series covering both of those, and also turning on tests

https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg02978.html

The series mentioned in comment #3 was merged in v2.12.0, see commit c21965a0c8b979c306e927f158257e5b0fa3a1f9.