fix: Support running git-[im,ex]port-patches with Python3 too (#32303)

* 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.

script/git-import-patches

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 
 import argparse
 import sys