Drop Python 3.4 and add 3.8

Python 3.4 reached end-of-life status nearly 4 months ago with the
release of 3.4.10 on 2019-03-18, so stop asserting we test new
commits against it (truth be told, it was already hard to build on
modern distros with newer libssl anyway).

Python 3.8 has been in beta for over a month already, and 3.8.0b2
was tagged earlier this week. New commits have been tested against
3.8 alpha and beta builds for a while, so add it to the list of
trove classifiers.

Update the README file to stop mentioning minimum Python 3 versions,
so that it won't need further updates as older interpreters age out.