You can add the following to your .vimrc and try:
autocmd BufRead,BufNewFile *.waf set filetype=python
If it is a particular filename like wscript, this works too:
autocmd BufRead,BufNewFile wscript set filetype=python
If you can't rely on an extension or filename you can add a modeline at the top or bottom of your file such as:
# vim: set filetype=python :