Python 3.8 has reached EOL and this PR removes old toolchains and most
of the tests. Users can still use it if they register the toolchains
themselves, but rules_python will no longer keep testing the toolchains.

Removing the toolchains all-together will be done at a later stage which
may require us to be more clever how we handle asks to include 3.8. Maybe
we can just fail if the user asks for a python version that does not exist,
but I am concerned that rules_python depending on protobuf may pull in
code that requests 3.8. I'll look at this at some later time.