importlib.readers not valid until python 3.10

This exists https://github.com/python/cpython/blob/3.10/Lib/importlib/readers.py and FileReader is in there
This is a 404 https://github.com/python/cpython/blob/3.9/Lib/importlib/readers.py

This change needs to get backported to the 7.0.z branch(s) too
Fixes pytest-dev#9608

src/_pytest/assertion/rewrite.py

@@ -273,7 +273,7 @@ def get_data(self, pathname: Union[str, bytes]) -> bytes:
         with open(pathname, "rb") as f:
             return f.read()
 
-    if sys.version_info >= (3, 9):
+    if sys.version_info >= (3, 10):
 
         def get_resource_reader(self, name: str) -> importlib.abc.TraversableResources:  # type: ignore
             from types import SimpleNamespace