Revert "github: disable pip caching temporarily"

This reverts commit 55d6cb4.

According to changelog setup-python v2.3.2 should include a workaround
for the issue.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: Jussi Kukkonen

.github/workflows/ci.yml

@@ -47,8 +47,8 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
-          # cache: 'pip'
-          # cache-dependency-path: 'requirements*.txt'
+          cache: 'pip'
+          cache-dependency-path: 'requirements*.txt'
 
       - name: Install dependencies
         run: |