bootstrap.py hardcodes Python binary

[TheCatPlusPlus]

On Windows py.exe launcher has been around for a long time, and python.exe shouldn't be in PATH. Which makes the build fail. Also it's a good idea to make hashbangs specify python2 or python3 explicitly (Windows launcher understands these too and switches accordingly).

Should have a way to specify the binary and/or use the full path from sys.executable.