title: AWS DynamoDB query fails when attribute have certain characters
body: <p>If I run this query:</p>
  
  <pre><code>data = table.query(KeyConditionExpression = Key('id').eq('fasfas'),
                     FilterExpression=Attr(u'Absolute humidity[g/kg].3').eq(1))
  </code></pre>
  
  <p>I get the error:</p>
  
  <pre><code>ClientError: An error occurred (ValidationException) when calling the Query operation: Invalid FilterExpression: Syntax error; token: "humidity", near: "Absolute humidity ["
  </code></pre>
  
  <p>However If I run the query using their website it works. How can I make it work using boto3?</p>
  
  <p>P.S. it also doens't work with underscores.</p>