Remove sys.modules hack

Fix #1888 (metadata accidentally not picklable), and removes a case where reimporting a vendored module results in a second copy of the same module.

Author: rotu

changelog.d/1890.change.rst

@@ -0,0 +1 @@
+Fix vendored dependencies so importing ``setuptools.extern.some_module`` gives the same object as ``setuptools._vendor.some_module``. This makes Metadata picklable again.

setuptools/extern/__init__.py

@@ -43,23 +43,16 @@ def load_module(self, fullname):
                 __import__(extant)
                 mod = sys.modules[extant]
                 sys.modules[fullname] = mod
-                # mysterious hack:
-                # Remove the reference to the extant package/module
-                # on later Python versions to cause relative imports
-                # in the vendor package to resolve the same modules
-                # as those going through this importer.
-                if sys.version_info >= (3, ):
-                    del sys.modules[extant]
                 return mod
             except ImportError:
                 pass
         else:
             raise ImportError(
-                "The '{target}' package is required; "
-                "normally this is bundled with this package so if you get "
-                "this warning, consult the packager of your "
-                "distribution.".format(**locals())
-            )
+                    "The '{target}' package is required; "
+                    "normally this is bundled with this package so if you get "
+                    "this warning, consult the packager of your "
+                    "distribution.".format(**locals())
+                )
 
     def install(self):
         """

setuptools/tests/test_extern.py

@@ -0,0 +1,22 @@
+import importlib
+import pickle
+
+from setuptools import Distribution
+from setuptools.extern import ordered_set
+from setuptools.tests import py3_only
+
+
+def test_reimport_extern():
+    ordered_set2 = importlib.import_module(ordered_set.__name__)
+    assert ordered_set is ordered_set2
+
+
+def test_orderedset_pickle_roundtrip():
+    o1 = ordered_set.OrderedSet([1, 2, 5])
+    o2 = pickle.loads(pickle.dumps(o1))
+    assert o1 == o2
+
+
+@py3_only
+def test_distribution_picklable():
+    pickle.loads(pickle.dumps(Distribution()))