Add tag-based versioning for git archive support

[brianv0]

This commit lets you use git tags to create new versions and makes it unnecessary to update setup.py when creating a new version.

This will let you get a tarball directly from github with enough information to install a proxmin version.

This will help with creating a standard conda-forge recipe, which I'd like to do.

As an example of what this PR can enable, try this script to install a fake 0.6.10 version, as tagged in my fork. (Note that the tag is currently in my fork - not in this repo)

export GITHUB_ORG="brianv0"
export TAG="0.6.10"
export PROXMINDIR=$(mktemp -d)
cd $PROXMINDIR
curl -LO https://github.com/${GITHUB_ORG}/proxmin/archive/$TAG.tar.gz
tar xzf $TAG.tar.gz
cd proxmin-$TAG
pip3 install .

This PR also contains a fix for python 3.8 syntax warning.