add inputs to dev tests workflow dispatch

.github/workflows/dev-testing.yml

@@ -1,6 +1,11 @@
 name: Dev Branch Testing
 on:
   workflow_dispatch:
+    inputs:
+      python-versions:
+        description: 'List of Python versions to use in matrix, as JSON string'
+        required: true
+        type: string
   workflow_call:
     inputs:
       python-versions: