* script: Python3 compatibility for utf8 conversion
The unicode() method has been renamed to str() in Python3,
add a wrapper around it to support running against both versions.
* script: don't require python2 for git-[import,export]-patches
The scripts work just fine with python3 too, so use the generic
python executable as the script interpreter.
Most setups don't even require or provide python 2 anymore,
so this saves one from having to install it just for the scripts.
