fix: add from __future__ import annotations to tests/conftest.py

Prevents collections.abc.Iterator[...] from being evaluated at runtime
on Python 3.8, where ABC types are not subscriptable.

Author: rembish

tests/conftest.py

@@ -1,5 +1,7 @@
 """Shared pytest fixtures for Miette tests."""
 
+from __future__ import annotations
+
 import warnings
 from collections.abc import Iterator