[BEAM-13657] Add python 3.6 update to CHANGES.md (#17435)
[BEAM-13657] Add python 3.6 update to CHANGES.md (#17435)

Co-authored-by: Brian Hulette <e5fce473ba473e73422afc5bb319c5cc358ed4c8@gmail.com>
lukecwik/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,apache/beam,apache/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam