community/py3-img2pdf: add missing dependency

py3-setuptools is required at runtime because /usr/bin/img2pdf includes the line:

from pkg_resources import load_entry_point

pkg_resources is a part of py3-setuptools:

$ apk info -W /usr/lib/python3.8/site-packages/pkg_resources/__init__.py
/usr/lib/python3.8/site-packages/pkg_resources/__init__.py is owned by py3-setuptools-46.1.3-r0

Before this change, img2pdf exits with the error below:

$ img2pdf --version
Traceback (most recent call last):
  File "/usr/bin/img2pdf", line 6, in <module>
    from pkg_resources import load_entry_point
ModuleNotFoundError: No module named 'pkg_resources'

After this change img2pdf succeeds:

$ img2pdf --version
img2pdf 0.3.4