main/py3-tappy: add missing dependency

Before this change:

$ dabuild sh
$ sudo apk add py3-tappy >/dev/null && tappy --help
Traceback (most recent call last):
  File "/usr/bin/tappy", 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/tappy
import pkg_resources

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-tappy >/dev/null && tappy --help
usage: tappy [-h] [-v] [FILE [FILE ...]]

A TAP consumer for Python

positional arguments:
  FILE           A file containing TAP output. Any directories listed will be scanned for files to
                 include as TAP files.

optional arguments:
  -h, --help     show this help message and exit
  -v, --verbose  use verbose messages

When no files are given or a dash (-) is used for the file name, tappy will read a TAP stream from
STDIN.