Ignore missing import.

I have no idea why pyre thinks this part of stdlib is missing...

src/antsibull_core/venv.py

@@ -10,7 +10,7 @@
 import asyncio
 import os
 import sys
-import venv
+import venv  # pyre-ignore[21]
 from collections.abc import Sequence
 from typing import TYPE_CHECKING, NoReturn