bpo-38469: Handle named expression scope with global/nonlocal keywords (GH-16755) (#16760)

The symbol table handing of PEP572's assignment expressions is not resolving correctly the scope of some variables in presence of global/nonlocal keywords in conjunction with comprehensions.
(cherry picked from commit fd5c414880b2e05720b9cf14ab0b0d7ae2b7d925)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
3 files changed