Thank you for your bug report.The version 3.05 of NSIS should result in a successful build via scons requiring Python 3 compatible syntax. In particular the included commit #7124 "Port SCons scripts to py3k (xantares/py3k PR)" [1] addresses that.
I am going to work on the upgrade of the NSIS Debian package to version 3.05.
[1] https://sourceforge.net/p/nsis/code/7124/
