Fix: Add pip dependencies caching to speed up workflow runs

.github/workflows/ubuntu20_python.yml

@@ -24,6 +24,13 @@ jobs:
       uses: actions/setup-python@v3
       with:
         python-version: ${{ matrix.python-version }}
+    - name: Cache dependencies
+      uses: actions/cache@v3
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-    
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip