Add Python 3.8 support

[mschwager]

We've recently included Python 3.8 in our CI system (although failures are allowed). The tests are currently failing on this version, see here.

We should fix the tests and add Python 3.8 support before it is officially released. It appears this version will be officially released sometime in October: PEP 569.

[mschwager]

See https://bugs.python.org/issue33211 and https://bugs.python.org/issue34876

[mschwager]

I believe this should be fixed in Python 3.8.0 beta 4, whereas TravisCI 3.8-dev appears to still be on 3.8.0 beta 3.

I misread the issues, per this PR (python/cpython#9731):

The lineno and col_offset attributes of the AST for decorated function
and class refer now to the position of the corresponding def, async def and class instead of the position of the first decorator. This
leads to more correct line reporting in tracing. This is the only case when
the position of child AST nodes can preceed the position of the parent AST
node.

We'll have to account for this discrepancy between 3.8+ and < 3.8.