Install also Python 3.7 for update-lint-requirements

Follow-up on commit d604bff .

Drop caching of pip directory for this workflow.

.github/workflows/dependencies.yaml

@@ -9,17 +9,17 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.8']  # poetry requires Python >=3.8
+        python-version: ['3.7']
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4
         with:
-          python-version: ${{ matrix.python-version }}
-      - name: Cache pip dir
-        uses: actions/cache@v3
-        with:
-          path: ~/.cache/pip
-          key: pip-cache-${{ matrix.python-version }}-${{ hashFiles('requirements.txt') }}
+          # poetry requires Python >=3.8, but lint requirements currently need
+          # to be generated with `pip freeze`` on the oldest Python version
+          # supported by Galaxy.
+          python-version: |
+            ${{ matrix.python-version }}
+            3.8
       - name: Update dependencies
         run: | 
           python -m venv .venv