Remove compat code for Python older than 3.8 from tests (#1010)

* Remove pre Python 3.8+ compat code from tests

* lint

tests/conftest.py

@@ -3,20 +3,12 @@
 import argparse
 import pickle
 from dataclasses import dataclass
+from functools import cached_property
 from importlib import import_module
 from sys import version_info as _version_info
 from types import ModuleType
 from typing import Callable, Type
 
-try:
-    from functools import cached_property  # Python 3.8+
-except ImportError:
-    from functools import lru_cache as _lru_cache
-
-    def cached_property(func):
-        return property(_lru_cache()(func))
-
-
 import pytest
 
 from multidict import MultiMapping, MutableMultiMapping