CI: Make tests-pypi a callable workflow

This is to make it reusable for Siphon.

.github/workflows/tests-pypi.yml

@@ -2,6 +2,7 @@ name: PyPI Tests
 
 # We don't want pushes (or PRs) to gh-pages to kick anything off
 on:
+  workflow_call:
   push:
     branches:
       - main