require python3 in standalone bundle
require python3 in standalone bundle

The shebang `/usr/bin/env python` may execute the bundle with Python 2
which leads to a import error later in the code execution:

```console
$ ./crash_standalone_latest
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "./crash_standalone_latest/__main__.py", line 11, in <module>
  File "./crash_standalone_latest/crate/crash/command.py", line 36, in <module>
  File "./crash_standalone_latest/crate/client/__init__.py", line 22, in <module>
  File "./crash_standalone_latest/crate/client/connection.py", line 24, in <module>
  File "./crash_standalone_latest/crate/client/http.py", line 39, in <module>
ImportError: No module named parse
```

The new shebang will make the give a better message if Python 3 is not
available:

```console
$ ./crash_standalone_latest
/usr/bin/env: ‘python3’: No such file or directory
```

crate/crash,crate/crash