input: Write a python function to establish an SSH connection using Paramiko.
  assert establish_ssh_connection("[REMOTEHOSTNAME]", "[USERNAME]", "[PASSWORD]") is not None
  assert establish_ssh_connection("invalid_host", "user", "pass") is None
output: def establish_ssh_connection(hostname, username, password):
      import paramiko
      client = paramiko.SSHClient()
      client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
      try:
          client.connect(hostname, username=username, password=password)
          return client
      except:
          return None