Use hashlib.sha256() instead of _sha256.sha256()

_sha256 is a CPython extension and is an implementation detail one must
not rely on. Despite being a part of most CPython distributions, _sha256
module may not exist if CPython has been built in so-called FIPS
compliant mode. Examples of such distributions could be found in CentOS
and/or RHEL.

Recommended way of using sha256() is via hashlib module because hashlib
module may choose a hash function implementation based on whether
OpenSSL exist or not. Normally _sha256 module is a fallback
implementation if CPython has not been linked to OpenSSL, but in FIPS
mode it's always linked to OpenSSL.

aiohttp_cache/backends.py

@@ -2,7 +2,7 @@
 import enum
 import pickle
 import time
-from _sha256 import sha256
+from hashlib import sha256
 from typing import Dict, Tuple
 
 import aiohttp.web