Stop trying to write into the stdlib during lib2to3 tests (#12331).

This prevents tests from failing when run from a Python installed in a
read-only directory.
diff --git a/Misc/NEWS b/Misc/NEWS
index 0aeddf4..01660d5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -161,6 +161,9 @@
 Tests
 -----
 
+- Issue #12331: The test suite for lib2to3 can now run from an installed
+  Python.
+
 - Issue #12549: Correct test_platform to not fail when OS X returns 'x86_64'
   as the processor type on some Mac systems.