[doc] The next tagged release is the last supporting Python 2

Dropping python 2.7 process was startet in January 2019.
Deprecation warning for both versons is shown after May 2019.
The next release is announced for July 2020 which is the last
supporting Python 2 and Python 3.4

Bug: T213287
Change-Id: Iaad78f5e5ae3b0f007c03a8a7dee898018470de2