Update readme

Author: russella-acm

README.md

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 # createsend
 
-A Python library which implements the complete functionality of the [Campaign Monitor API](http://www.campaignmonitor.com/api/). Requires Python 2.7, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9 or 3.10.
+A Python library which implements the complete functionality of the [Campaign Monitor API](http://www.campaignmonitor.com/api/). Requires Python 3.6, 3.7, 3.8, 3.9, 3.10, 3.11, 3.12, 3.13 or later.
 
 ## Installation