Add caching for ~/.cache/pip

.github/workflows/django.yml

@@ -51,6 +51,10 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+    - uses: actions/cache@v2
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-${{ matrix.python-version }}-pip-${{ hashFiles('**/requirements.txt') }}
     - name: Install Dependencies
       run: |
         python -m pip install --upgrade pip