tests: auto run tests when requirements-dev.txt changes

Earlier we added requirements.txt to the paths that trigger full tests
on PR and push, but that is not actually relevant, it is the -dev
version we need to run them on.

PR #3874

.github/workflows/tests.yml

@@ -5,11 +5,11 @@ on:
   pull_request:
     paths:
       - '**.py'
-      - 'requirements.txt'
+      - 'requirements-dev.txt'
   push:
     paths:
       - '**.py'
-      - 'requirements.txt'
+      - 'requirements-dev.txt'
 
 jobs:
   tests: