Adding concurrent futures copyright for EnhancedThreadPoolExecutor to LICENSE.txt

I'm making the copyright statement that is already in concurrent.futures.enhanced_thread_pool_executor more apparent by adding it to `LICENSE.txt`. I am not certain we are license compliant in any case. The code in question has been edited by us. Does anyone (@rkern, @itziakos, @sjagoe) know the proper license display/extension for PSF-contributed code?