Replace our tox-py37 job with tox-py38, extend the list of trove
classifiers for Python versions in package metadata, and replace the
"py35" in the tox.ini envlist with just "py3" so that folks running
`tox` unqualified on their systems will use whatever python3
interpreter they have on hand (odds are it's in our supported range
these days). Also uncap python-daemon so we use a version compatible
with Python >=3.8.