chore(CI): Add manually triggerable PyPI publication

We used to run the publication on each PR, which could cause conflicts
due to the naming convention being based on the commit history length,
and multiple PRs potentially hitting the same name.

Adding a `workflow_dispatch` trigger allows us to still test the
workflow in PRs but only do so manually (when we're watching anyway).

Changelog-None

.github/workflows/pypi.yml

@@ -3,6 +3,7 @@ name: Publish Python 🐍 distributions 📦 to PyPI and TestPyPI
 on:
   # Only deploy if we're the result of a PR being merged
   push:
+    workflow_dispatch:
     branches:
       - master
     tags: