Enable importlib.metadata backend on Python 3.11

Author: uranusjr

src/pip/_internal/metadata/__init__.py

@@ -1,7 +1,11 @@
+import contextlib
 import functools
 import os
+import sys
 from typing import TYPE_CHECKING, List, Optional, Type, cast
 
+from pip._internal.utils.misc import strtobool
+
 from .base import BaseDistribution, BaseEnvironment, FilesystemWheel, MemoryWheel, Wheel
 
 if TYPE_CHECKING:
@@ -22,14 +26,39 @@
 ]
 
 
+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 overriden by a couple of ways:
+
+    * If environment variable ``_PIP_USE_IMPORTLIB_METADATA`` is set, it
+      dictates whether ``importlib.metadata`` is used, regardless of Python
+      version.
+    * On Python 3.11+, Python distributors can patch ``importlib.metadata``
+      to add a global constant ``_PIP_USE_IMPORTLIB_METADATA = False``. This
+      makes pip use ``pkg_resources`` (unless the user set the aforementioned
+      environment variable to *True*).
+    """
+    with contextlib.suppress(KeyError, ValueError):
+        return bool(strtobool(os.environ["_PIP_USE_IMPORTLIB_METADATA"]))
+    if sys.version_info < (3, 11):
+        return False
+    try:
+        import importlib.metadata
+    except ImportError:
+        return True
+    return bool(getattr(importlib.metadata, "_PIP_USE_IMPORTLIB_METADATA", True))
+
+
 class Backend(Protocol):
     Distribution: Type[BaseDistribution]
     Environment: Type[BaseEnvironment]
 
 
 @functools.lru_cache(maxsize=None)
 def select_backend() -> Backend:
-    if os.environ.get("_PIP_METADATA_BACKEND_IMPORTLIB"):
+    if _should_use_importlib_metadata():
         from . import importlib
 
         return cast(Backend, importlib)