tests: run full tests on dependency update

Sometimes dependency updates cause test failures, but we don't know
immediately from the pull-request, since the full test suite is only
run when .py files are updated.

.github/workflows/tests.yml

@@ -5,9 +5,11 @@ on:
   pull_request:
     paths:
       - '**.py'
+      - 'requirements.txt'
   push:
     paths:
       - '**.py'
+      - 'requirements.txt'
 
 jobs:
   tests: