Reset the backports module when enabling vendored packages.

Closes #4476

newsfragments/4476.bugfix.rst

@@ -0,0 +1 @@
+Reset the backports module when enabling vendored packages.

pkg_resources/__init__.py

@@ -75,6 +75,8 @@
 import _imp
 
 sys.path.extend(((vendor_path := os.path.join(os.path.dirname(os.path.dirname(__file__)), 'setuptools', '_vendor')) not in sys.path) * [vendor_path])  # fmt: skip
+# workaround for #4476
+sys.modules.pop('backports', None)
 
 # capture these to bypass sandboxing
 from os import utime

ruff.toml

@@ -43,6 +43,11 @@ ignore = [
 	"ISC002",
 ]
 
+# Suppress nuisance warnings about E402 due to workaround for #4476
+[tool.ruff.lint.per-file-ignores]
+"setuptools/__init__.py" = ["E402"]
+"pkg_resources/__init__.py" = ["E402"]
+
 [format]
 # Enable preview to get hugged parenthesis unwrapping
 preview = true

setuptools/__init__.py

@@ -7,6 +7,8 @@
 from typing import TYPE_CHECKING
 
 sys.path.extend(((vendor_path := os.path.join(os.path.dirname(os.path.dirname(__file__)), 'setuptools', '_vendor')) not in sys.path) * [vendor_path])  # fmt: skip
+# workaround for #4476
+sys.modules.pop('backports', None)
 
 import _distutils_hack.override  # noqa: F401
 import distutils.core