use python3 only for building

Python 2 is very near end-of-life, and Python3-compatible changes to a
few scripts introduced compatibility problems with 2.7 again.  It went
unnoticed for me since my system symlinks "python" to "python3", but
it broke the build on systems where that symlink is still python2.  At
this point in time, I feel it is worth targetting modern Python and
forgetting about 2.7.

bootstrap/bootstrap

@@ -1,6 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
-from __future__ import print_function
 import sys
 import struct