Check the lockfile is version 2.0

Might one day incorporate other checks like #14742

.ci/scripts/check_lockfile.py

@@ -0,0 +1,23 @@
+#! /usr/bin/env python
+import sys
+
+if sys.version_info < (3, 11):
+    raise RuntimeError("Requires at least Python 3.11, to import tomllib")
+
+import tomllib
+
+with open("poetry.lock", "rb") as f:
+    lockfile = tomllib.load(f)
+
+try:
+    lock_version = lockfile["metadata"]["lock-version"]
+    assert lock_version == "2.0"
+except Exception:
+    print(
+        """\
+    Lockfile is not version 2.0. You probably need to upgrade poetry on your local box
+    and re-run `poetry lock --no-update`. See the Poetry cheat sheet at
+    https://matrix-org.github.io/synapse/develop/development/dependencies.html
+    """
+    )
+    raise

.github/workflows/tests.yml

@@ -51,6 +51,14 @@ jobs:
       - run: "pip install 'click==8.1.1' 'GitPython>=3.1.20'"
       - run: scripts-dev/check_schema_delta.py --force-colors
 
+  check-lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.x"
+      - run: .ci/scripts/check_lockfile.py
   lint:
     uses: "matrix-org/backend-meta/.github/workflows/python-poetry-ci.yml@v2"
     with:
@@ -163,6 +171,7 @@ jobs:
       - lint-pydantic
       - check-sampleconfig
       - check-schema-delta
+      - check-lockfile
       - lint-clippy
       - lint-rustfmt
     runs-on: ubuntu-latest