Fix "lint" and "providedBy" build errors

linter_plugin.py

@@ -0,0 +1,20 @@
+"""Let's Encrypt ACME PyLint plugin.
+
+http://docs.pylint.org/plugins.html
+
+"""
+from astroid import MANAGER
+from astroid import nodes
+
+
+def register(unused_linter):
+    """Register this module as PyLint plugin."""
+
+def _transform(cls):
+    if (('Message' in cls.basenames or 'ImmutableMap' in cls.basenames or
+         'util.ImmutableMap' in cls.basenames) and (cls.slots() is not None)):
+        for slot in cls.slots():
+            cls.locals[slot.value] = [nodes.EmptyNode()]
+
+
+MANAGER.register_transform(nodes.Class, _transform)