Fixes a pathological situation where if a function signature used a trailing
comma but was later reformatted to a single line (with the trailing comma
removed), Black would change its mind whether a file is Python
3.6-compatible between runs.
* Normalize string quotes
Convert single-quoted strings to double-quoted. Convert triple single-quoted strings to triple double-quoted. Do not touch any strings where conversion would increase the number of backslashes.
Fixes#51.
* reformat Black itself
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
