ci: only deploy to PyPI on X.Y.Z{,rcN} tags

We want to reserve other tags for our own purposes without it creating a
release.

Author: bluetech

.github/workflows/main.yml

@@ -6,7 +6,8 @@ on:
       - master
       - "[0-9]+.[0-9]+.x"
     tags:
-      - "*"
+      - "[0-9]+.[0-9]+.[0-9]+"
+      - "[0-9]+.[0-9]+.[0-9]+rc[0-9]+"
 
   pull_request:
     branches: