Remove reference to utcnow

This cleans up some of the datetime handling in the self check.

Note that this changes the format of the state file, since the datetime
now uses ``.isoformat()`` instead of ``.strftime``. Reading an outdated
state file will still work on Python 3.11+, but not on earlier versions.

src/pip/_internal/self_outdated_check.py

@@ -28,8 +28,8 @@
 from pip._internal.utils.filesystem import adjacent_tmp_file, check_path_owner, replace
 from pip._internal.utils.misc import ensure_dir
 
-_DATE_FMT = "%Y-%m-%dT%H:%M:%SZ"
 
+_WEEK = datetime.timedelta(days=7)
 
 logger = logging.getLogger(__name__)
 
@@ -76,9 +76,9 @@ def get(self, current_time: datetime.datetime) -> Optional[str]:
         seven_days_in_seconds = 7 * 24 * 60 * 60
 
         # Determine if we need to refresh the state
-        last_check = datetime.datetime.strptime(self._state["last_check"], _DATE_FMT)
-        seconds_since_last_check = (current_time - last_check).total_seconds()
-        if seconds_since_last_check > seven_days_in_seconds:
+        last_check = datetime.datetime.fromisoformat(self._state["last_check"])
+        time_since_last_check = (current_time - last_check)
+        if time_since_last_check > _WEEK:
             return None
 
         return self._state["pypi_version"]
@@ -100,7 +100,7 @@ def set(self, pypi_version: str, current_time: datetime.datetime) -> None:
             # Include the key so it's easy to tell which pip wrote the
             # file.
             "key": self.key,
-            "last_check": current_time.strftime(_DATE_FMT),
+            "last_check": current_time.isoformat(),
             "pypi_version": pypi_version,
         }
 
@@ -229,7 +229,7 @@ def pip_self_version_check(session: PipSession, options: optparse.Values) -> Non
     try:
         upgrade_prompt = _self_version_check_logic(
             state=SelfCheckState(cache_dir=options.cache_dir),
-            current_time=datetime.datetime.utcnow(),
+            current_time=datetime.datetime.now(datetime.timezone.utc),
             local_version=installed_dist.version,
             get_remote_version=functools.partial(
                 _get_current_remote_pip_version, session, options