docs: def get_signer_by_version(digest, ver):
      """Returns a new signer object for a digest and version combination.
  
      Keyword arguments:
      digest -- a callable that may be passed to the initializer of any Signer object in this library.
          The callable must return a hasher object when called with no arguments.
      ver -- the version of the signature. This may be any value convertible to an int.
      """
      if int(ver) == 1:
          return V1Signer(digest)
      elif int(ver) == 2:
          return V2Signer(digest)
      else:
          return None