Disable travis builds on 3.8 (#6912)
The new argument to PyCode_New means that lxml doesn't work on 3.8
right now, so we're out of business until there's a new lxml release
that does. (If that is taking too long, we can disable the tests that
want lxml.)
There are also some other minor issues: we fail some tests with
`DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead` while running some of the eval tests.
