tidy: enforce abstract methods have ellipsis body

[reidpr]

Split from #1860. Pylint: W2301 unnecessary-ellipsis

charliecloud/lib/registry.py

Lines 81 to 86 in d679373

	@classmethod
	def authenticate(class_, creds, auth_d):
	"""Authenticate using the given credentials and parsed WWW-Authenticate
	dictionary. Return a new Auth object if successful, None if
	not. The caller is responsible for dealing with the failure."""
	...

Like pass, I like the ellipsis here to make the empty body more clear. However, in this case the true error is missing @abc.abstractmethod. Can we enforce that abstract methods have an ellipsis body and vice versa?