Defer setuptools import to save ~60ms on startup

(I ran this 5 times and picked a median result)

### before

```console
$ time flake8 --help > /dev/null

real    0m0.221s
user    0m0.199s
sys     0m0.020s
```

### after

```console
$ time flake8 --help > /dev/null

real    0m0.169s
user    0m0.144s
sys     0m0.020s
```