Update supported Python versions.

 - (Removed) Python 2.7 was EOL on Jan 1st, 2020.
 - (Removed) Python 3.5 was EOL on Sep 13th, 2019.
 - Python 3.6 will end in 23 Dec 2021.
 - Python 3.7 will end in 27 Jun 2023.
 - (Adding) Python 3.8 released 14th Oct 2019.
 - (Adding) Python 3.9 released 05th Oct 2020.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>