Added requirements files to cache-dependency-path

Multiple packages installed during some of the somewhat recent workflow
runs were not fetched from cache, as the pinned package versions were
updated in requirements files, and not in `setup.py` or `tox.ini` - which
caused the cached package versions to remain unchanged after updating
the pinned versions.

Author: ddabble

.github/workflows/test.yml

@@ -69,6 +69,7 @@ jobs:
           cache-dependency-path: |
             setup.py
             tox.ini
+            requirements/*.txt
 
       - name: Install dependencies
         run: |