Fix DynamoDb2 ExpressionAttributeNames can start with a number (#3206)

When using pynamodb's support for transactions it makes use of of
ExpressionAttributeNames that look like #0 #1 etc. According to

https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ExpressionAttributeNames.html

And when testing against dynamodb-local these work without issue,
however, when testing with moto they fail.

moto/dynamodb2/parsing/tokens.py

@@ -109,7 +109,7 @@ class ExpressionTokenizer(object):
 
     @classmethod
     def is_expression_attribute(cls, input_string):
-        return re.compile("^[a-zA-Z][a-zA-Z0-9_]*$").match(input_string) is not None
+        return re.compile("^[a-zA-Z0-9][a-zA-Z0-9_]*$").match(input_string) is not None
 
     @classmethod
     def is_expression_attribute_name(cls, input_string):