Remove shebang from cli.py

[mtelka]

The shebang is useless in the cli.py file:

$ PYTHONPATH=. dulwich/cli.py 
Traceback (most recent call last):
  File "/tmp/dulwich/dulwich/cli.py", line 41, in <module>
    from .client import GitProtocolError, get_transport_and_path
ImportError: attempted relative import with no known parent package
$

So let's remove it.