main/py3-future: add missing dependency

Before this change:

$ dabuild sh
$ sudo apk add py3-future >/dev/null && futurize --version
Traceback (most recent call last):
  File "/usr/bin/futurize", 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/futurize
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-future >/dev/null && futurize --version
0.18.2