Fix BytesDecoder

Author: kclowes

web3/_utils/abi.py

@@ -227,7 +227,6 @@ class ExactLengthBytesEncoder(encoding.BaseEncoder):
     value_bit_size = None
     data_byte_size = None
     encode_fn = None
-    is_dynamic = True
 
     def validate(self):
         super().validate()
@@ -304,7 +303,6 @@ def from_type_str(cls, abi_type, registry):
 
 class BytesDecoder(decoding.FixedByteSizeDecoder):
     is_big_endian = False
-    is_dynamic = True
 
     @staticmethod
     def decoder_fn(data):