bpo-27946: Fix possible crash in ElementTree.Element (GH-29915)

Getting an attribute via attrib.get() simultaneously with replacing
the attrib dict can lead to access to deallocated dict.
(cherry picked from commit d15cdb2f32f572ce56d7120135da24b9fdce4c99)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>