Add `SSZVariableName` custom type

scripts/build_spec.py

@@ -55,6 +55,7 @@ from eth2spec.utils.ssz.ssz_impl import (
 )
 from eth2spec.utils.ssz.ssz_typing import (
     BasicValue, Elements, BaseList, SSZType,
+    SSZVariableName,
     Container, List, Vector, Bytes, BytesN, Bitlist, Bitvector, Bits,
     Bytes1, Bytes4, Bytes8, Bytes32, Bytes48, Bytes96,
     uint64, bit, boolean,

test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py

@@ -1,8 +1,11 @@
-from typing import Dict, Iterator
+from typing import Dict, Iterator, NewType
 import copy
 from types import GeneratorType
 
 
+SSZVariableName = NewType('SSZVariableName', str)
+
+
 class DefaultingTypeMeta(type):
     def default(cls):
         raise Exception("Not implemented")