CI: Use common linting workflow

After #12107 it's much easier for black, isort and flake8 to find the
scripts we want them to lint.

.github/workflows/tests.yml

@@ -20,20 +20,11 @@ jobs:
       - run: scripts-dev/config-lint.sh
 
   lint:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        toxenv:
-          - "check_codestyle"
-          - "check_isort"
-          - "mypy"
-          - "packaging"
-
-    steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-python@v2
-      - run: pip install tox
-      - run: tox -e ${{ matrix.toxenv }}
+    # This does a vanilla `poetry install` - no extras. I'm slightly anxious
+    # that we might skip some typechecks on code that uses extras. However,
+    # I think the right way to fix this is to turn up mypy's strictness:
+    # disallow unknown imports and be accept fewer uses of `Any`.
+    uses: "matrix-org/backend-meta/.github/workflows/python-poetry-ci.yml@v1"
 
   lint-crlf:
     runs-on: ubuntu-latest