prep for PR to upstream

.github/workflows/downstream_testing.yml

@@ -1,6 +1,11 @@
 name: downstream_testing_2
 
 on:
+  workflow_dispatch:
+  pull_request:
+    branches:
+      # Only run downstream tests on PRs for releases
+      - "[0-9]+.[0-9]+.x"
   push:
     branches:
       - downstream_testing_2

changelog/7342.trivial.rst

@@ -0,0 +1,2 @@
+Added a system to run downstream plugin tests against current pytest, using GitHub Actions. Currently 
+constrained to pytest plugins that utilize GitHub Actions and ``tox`` to accomplish their own tests.