Per Debian policy, Python versions 3 and greater must be called from the
`python3` executable, not the `python` executable. This means
Debian-based systems with Python 3 installed but not Python 2 will fail
to launch SET through `seautomate` because the `python` executable is
hard-coded into the call to `pexpect.spawn()`.
This commit uses the standard library's
`distutils.spawn.find_executable()` method to locate the correct path to
`python3` or, failing that, the correct path to the `python` executable
and uses the result of that call as the first command line word with
which to invoke `setoolkit`.
The `distutils.spawn.find_executable()` method is available at that
exact name in both Python 2 and 3, so this should be portable across all
Python versions.
Note that the shebang line references `python`, and so Python 3-only
systems such as newer Debian-based builds may need to invoke
`seautomate` using an explicit interpreter for it to work:
```sh
sudo python3 ./seautomate /path/to/script/file.txt
```
