Stop trying to write into the stdlib during packaging tests (#12331).

This prevents tests from failing when run from a Python installed in a
read-only directory.  The code is a bit uglier; shutil.copytree calls
copystat on directories behind our back, so I had to add an os.walk
with os.chmod (*and* os.path.join!) calls.  shutil, I am disappoint.

This changeset is dedicated to the hundreds of neurons that were lost
while I was debugging this on an otherwise fine afternoon.
diff --git a/Misc/NEWS b/Misc/NEWS
index 3f5aa29..afe4894 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1147,6 +1147,9 @@
 Tests
 -----
 
+- Issue #12331: The test suite for the packaging module can now run from an
+  installed Python.
+
 - Issue #12331: The test suite for lib2to3 can now run from an installed
   Python.