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):

tests/test_dynamodb2/test_dynamodb_expression_tokenizer.py

@@ -219,6 +219,18 @@ def test_expression_tokenizer_single_set_action_attribute_name_valid_key():
     ]
 
 
+def test_expression_tokenizer_single_set_action_attribute_name_leading_number():
+    set_action = "SET attr=#0"
+    token_list = ExpressionTokenizer.make_list(set_action)
+    assert token_list == [
+        Token(Token.ATTRIBUTE, "SET"),
+        Token(Token.WHITESPACE, " "),
+        Token(Token.ATTRIBUTE, "attr"),
+        Token(Token.EQUAL_SIGN, "="),
+        Token(Token.ATTRIBUTE_NAME, "#0"),
+    ]
+
+
 def test_expression_tokenizer_just_a_pipe():
     set_action = "|"
     try: