ci: update setting oldest (#7)

Author: Borda

.github/workflows/ci-testing.yml

@@ -16,9 +16,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macOS-latest, windows-latest]
-        python-version: [3.9]
-        requires: ["oldest", "latest"]
+        os: ["ubuntu-latest", "macOS-latest", "windows-latest"]
+        python-version: ["3.9", "3.12"]
+        requires: ["latest"]
+        include:
+          - { os: "ubuntu-20.04", python-version: "3.9", requires: "oldest" }
 
     # Timeout: https://stackoverflow.com/a/59076067/4521646
     timeout-minutes: 35
@@ -31,31 +33,13 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-
-      # Github Actions: Run step on specific OS: https://stackoverflow.com/a/57948488/4521646
-      #- name: Setup macOS
-      #  if: runner.os == 'macOS'
-      #  run: |
-      #    brew install libomp  # https://github.com/pytorch/pytorch/issues/20030
+          cache: "pip"
 
       - name: Set min. dependencies
         if: matrix.requires == 'oldest'
         run: |
-          for fpath in ('requirements.txt', '_requirements/test.txt'):
-              req = open(fpath).read().replace('>=', '==')
-              open(fpath, 'w').write(req)
-        shell: python
-
-      - name: Get pip cache dir
-        id: pip-cache
-        run: echo "dir=$(pip cache dir)" >> $GITHUB_OUTPUT
-
-      - name: Cache pip
-        uses: actions/cache@v4
-        with:
-          path: ${{ steps.pip-cache.outputs.dir }}
-          key: ${{ runner.os }}-${{ matrix.python-version }}-${{ matrix.requires }}-pip-${{ hashFiles('requirements.txt') }}
-          restore-keys: ${{ runner.os }}-${{ matrix.python-version }}-${{ matrix.requires }}-pip-
+          pip install 'lightning-utilities[cli]'
+          python -m lightning_utilities.cli requirements set-oldest --req_files='["requirements.txt"]'
 
       - name: Install package & dependencies
         run: |