mypy

848a78d1 - PEP 570: Positional-Only Arguments (#6900)

PEP 570: Positional-Only Arguments (#6900) Fixes #6878. This was actually much more straightforward than I anticipated; since we already support `__some` forms of positional-only arguments, it was just a matter of adding a couple of lines to properly pick up the new `posonlyargs` attribute from the AST. Skip PEP 570 tests until Travis supports 3.8.0a4+.