Are you asking about Pyzotero (I'm the maintainer), or about the API v3 functionality in general?
If it's about Pyzotero, you'll be able to use the API v3-compatible version when I release it sometime in September (or you can test it now by checking out the api_v3 branch from Github and installing it). You'd be using the search functionality (
https://www.zotero.org/support/dev/web_api/v3/basics#search_syntax) to retrieve items relating to a tag. For example, if you wanted to retrieve the last five top-level-items tagged "Beckett", the call would be zot.top(tag='Beckett", limit=5). If you wanted your results CSL-formatted, you'd additionally specify e.g. style='chicago'.