Describe the bug

Couple tools are still using six which is no longer a dependency and not needed with Python 3 only code.

To Reproduce

In an environment without six, you will get:

ModuleNotFoundError: No module named 'six'

Expected behavior

We probably have six lingering here and there in the CI and any dependency can bring six as a transitive dependency for us, but removing usage of six from our code is the first, and possibly the only, step to do.

Screenshots

grep -IrnE "from six|import six"

src/raster/r.in.srtm.region/r.in.srtm.region.py:125:from six.moves.urllib import request as urllib2
src/raster/r.in.nasadem/r.in.nasadem.py:119:from six.moves.urllib import request as urllib2
src/raster/r.in.usgs/r.in.usgs.py:159:from six.moves.urllib.request import urlopen
src/raster/r.in.usgs/r.in.usgs.py:160:from six.moves.urllib.error import URLError, HTTPError
src/raster/r.in.usgs/r.in.usgs.py:161:from six.moves.urllib.parse import quote_plus

Additional context

I found this through the addon build logs artifact in the new Documentation workflow in the main repo.