This makes it consistent with removing the trailing comma when multiple
arguments to a call fit in a single line. It also makes it a tiny bit more
likely that an expression will fit a line that didn't use to.
Trailing commas after * or ** in a function signature are only safe for Python 3.6
code. So now Black checks whether the file was already Python 3.6 to begin
with. If so, trailing commas are used in such cases. Otherwise, they're not.
When * and ** don't appear in a function signature, the trailing comma is
always safe.
Fixes#8
