build: update prerequisites on progress towards Python 3

As recommended at https://github.com/nodejs/node/pull/25759#issuecomment-458139766
* Python 2.6 end of life statement in 2013: https://www.python.org/dev/peps/pep-0361/#release-lifespan
* Python 2.7 end of life statement in 2019: https://www.python.org/dev/peps/pep-0373/#update
* Python 3.4 reaches it end of life in < 50 days so it should not be a target: https://devguide.python.org/#branchstatus

PR-URL: https://github.com/nodejs/node/pull/25766


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>