adding caching of dependencies

.github/workflows/tests.yml

@@ -25,6 +25,15 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
+    - name: Cache dependencies
+      uses: actions/cache@v2
+      with:
+        path: |
+          ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements*.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+
     - name: Install dependencies
       run: |
         sudo apt-get install -y libopenmpi-dev