install_script with develop command broken on Python 3

install_script with the develop command uses the execfile() function, which doesn't exist in Python 3.
The proper replacement for {{{execfile(filename)}}}
is something like
{{{exec(compile(open(filename).read(), filename, 'exec'))}}}.

PS : this is with 0.6.24, but it's not in the "Version" drop-down.

Now load legacy scripts wrappers from templates in the package, which get converted to Python 3 syntax when built on Python 3. Fixes #273.

→ 6124053afb5c

Looks like this was previously-reported in #220