CI: Use common linting workflow

- We don't recommend tox in `code_style.md`
- After #12107 it's much easier for black, isort and flake8 to find the
  scripts we want them to lint; we don't need to maintain `lint_targets`
  in the tox config. See also #12420.

.github/workflows/tests.yml

@@ -20,19 +20,13 @@ jobs:
       - run: scripts-dev/config-lint.sh
 
   lint:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        toxenv:
-          - "check_codestyle"
-          - "check_isort"
-          - "mypy"
-
-    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 mark any extras needed for
+    # typechecking as development dependencies. To detect this, we ought 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

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37, py38, py39, py310, check_codestyle, check_isort
+envlist = py37, py38, py39, py310
 
 # we require tox>=2.3.2 for the fix to https://github.com/tox-dev/tox/issues/208
 minversion = 2.3.2
@@ -32,20 +32,6 @@ deps =
     # install the "enum34" dependency of cryptography.
     pip>=10
 
-# directories/files we run the linters on.
-# TODO: this is now out of date; we will remove as part of poetry migration.
-lint_targets =
-    setup.py
-    synapse
-    tests
-    # annoyingly, black doesn't find these so we have to list them
-    scripts-dev
-    stubs
-    contrib
-    synmark
-    .ci
-    docker
-
 # default settings for all tox environments
 [testenv]
 deps =
@@ -138,18 +124,3 @@ setenv =
 commands =
     python -m synmark {posargs:}
 
-[testenv:check_codestyle]
-extras = lint
-commands =
-    python -m black --check --diff {[base]lint_targets}
-    flake8 {[base]lint_targets} {env:PEP8SUFFIX:}
-
-[testenv:check_isort]
-extras = lint
-commands = isort -c --df {[base]lint_targets}
-
-[testenv:mypy]
-deps =
-    {[base]deps}
-extras = all,mypy
-commands = mypy