Merge pull request #78 from davesque/fix-mistake

Fix typo introduced in PR #76

eth_abi/base.py

@@ -144,4 +144,4 @@ def from_type_str(cls, type_str, registry):  # pragma: no cover
         Used by ``ABIRegistry`` to get an appropriate encoder or decoder
         instance for the given type string and type registry.
         """
-        raise cls()
+        raise NotImplementedError('Must implement `from_type_str`')