Drop Python 2.7, 3.5 support, add 3.6-3.9 for fluent.runtime, fluent.pygments (#163)

* Declare compat with current python versions, drop 2.7, 3.5 Also adjust testing matrix to that.
* Drop six, but keep it installed in tests
* Use pyupgrade --py36-plus to modernize syntax.
* Update version number and CHANGELOG
* Add six to install dependencies, because fluent.syntax didn't.
  * Also dropping dependencies on mock, which I didn't spot in the updates
via pyupgrade.