Run Tests workflow on v3-backport branch

The Tests.yml workflow was only triggering on PRs to master, so PRs
targeting v3-backport had no CI coverage. Add v3-backport to the
branches list for both pull_request and push triggers.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

.github/workflows/Tests.yml

@@ -4,11 +4,13 @@ on:
   pull_request:
     branches:
       - master
+      - v3-backport
     paths-ignore:
       - 'docs/**'
   push:
     branches:
       - master
+      - v3-backport
     paths-ignore:
       - 'docs/**'