[docs] Please update PEP 596 with the list of new features in Python 3.9 (at last)

Hi there! First of all, sorry if it's not the right mailing list to ask about it. I'm not sure which of them covers changes in such PEP documents. These "Release Schedule" informational PEPs used to be a handy concise reference of which feature PEPs are scheduled for a particular language version. Now, as Python 3.9 is in development, this information has to be recollected from dev documentation/mailing lists/newsletters/twits, etc. It can get confusing at times since not every accepted PEP ultimately goes into the next language version. Please keep them up to date, they are very helpful.