Experimental: run CI/CD checks only on changed Python/YAML files

It doesn't make sense to run data transformation tests on changes
in Jinja2 templates or documentation. Let's hope the 'paths'
parameter is what's needed to get that done.

Author: ipspace

.github/workflows/tests.yml

@@ -3,7 +3,13 @@ name: Run CICD Tests
 on:
   push:
     branches-ignore: [ devc ]
+    paths:
+    - '**.py'
+    - '**.yaml'
   pull_request:
+    paths:
+    - '**.py'
+    - '**.yaml'
 
 jobs:
   tests: