chore: update pylint settings to match changes in 2.5.0 (#265)

* chore: update pylint settings to match changes in 2.5.0

* docs: add explanation of why validator function signature is what it is

* chore: add full-message-decrypt scenario name into failing error message

Author: mattsb42-aws

src/aws_encryption_sdk/internal/validators.py

@@ -11,7 +11,10 @@
     pass
 
 
-def value_is_not_a_string(instance, attribute, value):
+# The unused-argument check is disabled because
+# this function MUST match the function signature
+# for attrs validators.
+def value_is_not_a_string(instance, attribute, value):  # pylint: disable=unused-argument
     # type: (Any, attr.Attribute, Any) -> None
     """Technically a string is an iterable containing strings.
 

test_vector_handlers/src/awses_test_vectors/manifests/full_message/decrypt.py

@@ -157,7 +157,7 @@ def run(self, name):
         """
         plaintext, _header = aws_encryption_sdk.decrypt(source=self.ciphertext, key_provider=self.master_key_provider)
         if plaintext != self.plaintext:
-            raise ValueError("Decrypted plaintext does not match expected value.")
+            raise ValueError("Decrypted plaintext does not match expected value for scenario '{}'".format(name))
 
 
 @attr.s(init=False)