Returns whether the C extension is installed correctly.

def is_installed():
  # type: () -> bool
  """
  Returns whether the C extension is installed correctly.
  """
  try:
    # noinspection PyUnresolvedReferences
    from ccurl import Curl as CCurl
  except ImportError:
    return False
  else:
    # noinspection PyUnresolvedReferences
    from iota.crypto import Curl
    return issubclass(Curl, CCurl)