Run CI suite every Monday morning at 3am

This should help flag any issues with new releases of our dependencies
(when we've got unlocked upper bounds on our dependencies).

.github/workflows/tests.yaml

@@ -13,6 +13,8 @@ on:
       - 'docs/**'
       - '*.md'
       - '*.rst'
+  schedule:
+    - cron: '0 3 * * 1'
 jobs:
   tests:
     name: ${{ matrix.tox == 'normal' && format('py{0}', matrix.python) || matrix.tox }}