Fix 12-year-old FIXME

Comments in docs and code for Image.thumbnail(), dating back to PIL 1.1.3 from March 14, 2002:

> If omitted, it defaults to `PIL.Image.NEAREST` (this will be changed to `ANTIALIAS` in a future version).

```
# FIXME: the default resampling filter will be changed
        # to ANTIALIAS in future versions
```

It is now the future.

The resample parameter was added in PIL 1.1.2 (May 2001) and defaulted to ANTIALIAS. In 1.1.3 (March 2002) it was changed to NEAREST with the FIXME note.

No reason is given in the [PIL 1.1.3 release notes](http://effbot.org/zone/pil-changes-113.htm), but it looks like there were initially some problems with ANTIALIAS so they went with the safer option:

> (1.1.3c2 released)

>  * ...

>  * Fixed segmentation violation in ANTIALIAS filter (an internal buffer wasn’t properly allocated).

> (1.1.3c1 released)

> * Added ANTIALIAS downsampling filter for high-quality “resize” and “thumbnail” operations. Also added filter option to the “thumbnail” operation; the default value is NEAREST, but this will most likely change in future versions.

PR also contains some pep8 and pyflakes cleanup.

TODO:
 * [ ] Update the docs
 * [ ] Should there be a note in the docs saying the default was NEAREST before version 2.5.0?