Enable package cache

This commit enables persistence of the pip cache between different CI
jobs, which will further reduce the load on PyPI.

Author: diazona

.github/workflows/tests.yml

@@ -51,6 +51,7 @@ jobs:
       with:
         python-version: ${{ matrix.versions.python }}
         allow-prereleases: true
+        cache: 'pip'
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip