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.