Fix closes Issue12183 - Explain the Symlink copy behavior in shutil.copytree. Patch by Petri Lehtinen.

Reported by Popa Claudiu and Patch by Santoso Wijaya.

Reported by Popa Claudiu and Patch by Santoso Wijaya.

Also, add an entry for the script in Tools/scripts/README.

Issue #11049: fix test_forget to work on installed Python, by using a temporary module for import/forget

See: https://bugzilla.redhat.com/show_bug.cgi?id=726536

This will be particularly useful to Windows users.

run_tests.py originally written by Brett Cannon.

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.

This prevents tests from failing when run from a Python installed in a
read-only directory.

Code extracted from Victor Stinner's patch for issue #12567.