In `bin/steps/collectstatic` the unbuffered output in `indent` is subverted
by calling `sed` first:
```shell
python $MANAGE_FILE collectstatic --noinput 2>&1 | sed '/^Copying/d;/^$/d;/^ /d' | indent
```
This commit fixes this by making `sed` itself unbuffered rather than
putting that logic in the `indent` function.