Fix new dependency-groups feature to use the stdlib tomllib where possible

Previously, commit 88c9f31 modified pip
to use the stdlib on versions of python where this module is in the
stdlib. As justified there:

Although a tomli copy is vendored, doing this conditional import allows:
- automatically upgrading the code, when the time comes to drop py3.10
  support

- slightly simplifying debundling support, as it's no longer necessary
  to depend on a tomli(-wheel)? package on sufficiently newer versions
  of python.

#13065 added a new feature, including a
vendored "dependency_groups" library that likewise supports using the
stdlib tomllib via `dependency_groups/_toml_compat.py`. But the code in
pip itself to use dependency_groups manually loads pyproject.toml and
passes it to dependency_groups, and fails to use the same compatibility
dispatch as both the pre-existing pip code and dependency_groups itself.

Add back the conditional logic.

Author: eli-schwartz

news/13356.vendor.rst

@@ -0,0 +1 @@
+Fix issues with using tomllib from the stdlib if available, rather than tomli

src/pip/_internal/req/req_dependency_group.py

@@ -1,6 +1,11 @@
+import sys
 from typing import Any, Dict, Iterable, Iterator, List, Tuple
 
-from pip._vendor import tomli
+if sys.version_info >= (3, 11):
+    import tomllib
+else:
+    from pip._vendor import tomli as tomllib
+
 from pip._vendor.dependency_groups import DependencyGroupResolver
 
 from pip._internal.exceptions import InstallationError
@@ -65,10 +70,10 @@ def _load_pyproject(path: str) -> Dict[str, Any]:
     """
     try:
         with open(path, "rb") as fp:
-            return tomli.load(fp)
+            return tomllib.load(fp)
     except FileNotFoundError:
         raise InstallationError(f"{path} not found. Cannot resolve '--group' option.")
-    except tomli.TOMLDecodeError as e:
+    except tomllib.TOMLDecodeError as e:
         raise InstallationError(f"Error parsing {path}: {e}") from e
     except OSError as e:
         raise InstallationError(f"Error reading {path}: {e}") from e

tests/unit/test_req_dependency_group.py

@@ -120,7 +120,7 @@ def epipe_toml_load(*args: Any, **kwargs: Any) -> None:
         raise OSError(errno.EPIPE, "Broken pipe")
 
     monkeypatch.setattr(
-        "pip._internal.req.req_dependency_group.tomli.load", epipe_toml_load
+        "pip._internal.req.req_dependency_group.tomllib.load", epipe_toml_load
     )
 
     with pytest.raises(InstallationError, match=r"Error reading pyproject\.toml"):