Model SQLAlchemy 2.0 and 2.1 in the base-install test

tests/fixtures/base_install_worker.py

@@ -6,8 +6,15 @@ the ``instrumentation`` extra. Argument: a directory for the feedback database.
 """
 
 import sys
+from importlib.metadata import version
 
-for _name in ("greenlet", "aiosqlite"):
+# SQLAlchemy 2.1 refuses to import sqlalchemy.ext.asyncio without greenlet,
+# once: a second import succeeds. 2.0 imports it and fails on first use.
+# On 2.0 the module is blocked too, so both versions fail at import.
+_blocked = ["greenlet", "aiosqlite"]
+if tuple(int(p) for p in version("sqlalchemy").split(".")[:2]) < (2, 1):
+    _blocked.append("sqlalchemy.ext.asyncio")
+for _name in _blocked:
     sys.modules[_name] = None  # import raises ModuleNotFoundError
 
 import asyncio  # noqa: E402