Update GitHub Actions to supported versions

- Upgrade actions/checkout v2 → v4, actions/setup-python v2 → v5
- Replace deprecated actions/cache v1 with built-in pip caching
- Drop EOL Python versions (3.6, 3.7, 3.8) from test matrix

Author: lavr

.github/workflows/tests.yaml

@@ -20,27 +20,17 @@ jobs:
           - {name: '3.11', python: '3.11', os: ubuntu-latest, tox: py311}
           - {name: '3.10', python: '3.10', os: ubuntu-latest, tox: py310}
           - {name: '3.9', python: '3.9', os: ubuntu-latest, tox: py39}
-          - {name: '3.8', python: '3.8', os: ubuntu-latest, tox: py38}
-          - {name: '3.7', python: '3.7', os: ubuntu-latest, tox: py37}
-          - {name: '3.6', python: '3.6', os: ubuntu-20.04, tox: py36}
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-python@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python }}
+          cache: pip
       - name: update pip
         run: |
           pip install -U wheel
           pip install -U setuptools
           python -m pip install -U pip
-      - name: get pip cache dir
-        id: pip-cache
-        run: echo "::set-output name=dir::$(pip cache dir)"
-      - name: cache pip
-        uses: actions/cache@v1
-        with:
-          path: ${{ steps.pip-cache.outputs.dir }}
-          key: pip|${{ runner.os }}|${{ matrix.python }}|${{ hashFiles('setup.py') }}|${{ hashFiles('requirements/*.txt') }}
       - run: pip install tox
       - name: run rests
         env: