TOOLS/file2string: change to python3

The same was done to matroska.py before, so at least it's consistent.
Doesn't matter for waf, because it imports this script (rather than
executing it).

TOOLS/file2string.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Convert the contents of a file into a C string constant.
 # Note that the compiler will implicitly add an extra 0 byte at the end