main/py3-wheel: add missing dependency

Before this change:

$ dabuild sh
$ sudo apk add py3-wheel >/dev/null && wheel --help
Traceback (most recent call last):
  File "/usr/bin/wheel", line 6, in <module>
    from pkg_resources import load_entry_point
ModuleNotFoundError: No module named 'pkg_resources'

This change adds py3-setuptools as a runtime dependency because pkg_resources is imported by a file installed into /usr/bin/:

$ grep pkg_resources /usr/bin/wheel
from pkg_resources import load_entry_point

pkg_resources is part of py3-setuptools:

$ apk info -L py3-setuptools | grep pkg_resources/__init__.py
usr/lib/python3.8/site-packages/pkg_resources/__init__.py

After this change:

$ dabuild sh
$ sudo apk add py3-wheel >/dev/null && wheel -h
usage: wheel [-h] {unpack,pack,convert,version,help} ...

positional arguments:
  {unpack,pack,convert,version,help}
                        commands
    unpack              Unpack wheel
    pack                Repack wheel
    convert             Convert egg or wininst to wheel
    version             Print version and exit
    help                Show this help

optional arguments:
  -h, --help            show this help message and exit