Add poetry caching to pytest

.github/workflows/pytest.yml

@@ -27,6 +27,11 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Set up Poetry
       uses: Gr1N/setup-poetry@v8
+    - name: Cache Poetry virtualenv
+      uses: actions/cache@v2
+      with:
+        path: ~/.cache/pypoetry/virtualenvs
+        key: ${{ runner.os }}-poetry-${{ hashFiles('**/poetry.lock') }}
     - name: Check Poetry config
       run: poetry check
     - name: Install dependencies