Silently ignore pkg_resources override on 3.12+

This customization is generally provided by the distributor, and the
user can do nothing if the environment is configured incorrectly. So
instead of breaking pip entirely, we silently ignore the customization
and let the distributor correct the situation.

news/11501.process.rst

@@ -1,6 +1,6 @@
-Emit an error to prevent the ``pkg_resources`` metadata backend from being used
-on Python 3.12 or later. This backend exists for backward compatibility, and
-should only be used on newer Python versions by Python distributors that cannot
-correctly implement ``importlib.metadata`` support in a timely fashion. Time is
-up for transition since CPython is now removing mechanisms that ``pkg_resource``
+Prevent the ``pkg_resources`` metadata backend from being used on Python 3.12
+or later. This backend exists for backward compatibility, and should only be
+used on newer Python versions by Python distributors that cannot correctly
+implement ``importlib.metadata`` support in a timely fashion. Time is up for
+transition since CPython is now removing mechanisms that ``pkg_resource``
 relies on, and the backend will no longer work on Python 3.12 or later.

src/pip/_internal/metadata/__init__.py

@@ -4,7 +4,6 @@
 import sys
 from typing import TYPE_CHECKING, List, Optional, Type, cast
 
-from pip._internal.exceptions import PipError
 from pip._internal.utils.misc import strtobool
 
 from .base import BaseDistribution, BaseEnvironment, FilesystemWheel, MemoryWheel, Wheel
@@ -31,7 +30,8 @@ def _should_use_importlib_metadata() -> bool:
     """Whether to use the ``importlib.metadata`` or ``pkg_resources`` backend.
 
     By default, pip uses ``importlib.metadata`` on Python 3.11+, and
-    ``pkg_resourcess`` otherwise. This can be overridden by a couple of ways:
+    ``pkg_resources`` otherwise. On Python versions prior to 3.12, this can be
+    overridden by a couple of ways for transitional purposes:
 
     * If environment variable ``_PIP_USE_IMPORTLIB_METADATA`` is set, it
       dictates whether ``importlib.metadata`` is used, regardless of Python
@@ -41,6 +41,8 @@ def _should_use_importlib_metadata() -> bool:
       makes pip use ``pkg_resources`` (unless the user set the aforementioned
       environment variable to *True*).
     """
+    if sys.version_info >= (3, 12):
+        return True
     with contextlib.suppress(KeyError, ValueError):
         return bool(strtobool(os.environ["_PIP_USE_IMPORTLIB_METADATA"]))
     if sys.version_info < (3, 11):
@@ -61,11 +63,6 @@ def select_backend() -> Backend:
         from . import importlib
 
         return cast(Backend, importlib)
-
-    if sys.version_info >= (3, 12):
-        message = "Cannot use pkg_resources as metadata backend on Python 3.12+"
-        raise PipError(message)
-
     from . import pkg_resources
 
     return cast(Backend, pkg_resources)