Fix test_ast_expr for Python 3.13

[thunze]

Python 3.13 introduced the show_empty keyword argument to ast.dump. If show_empty is False, ast.dump omits empty lists and fields that are None from its output.

Since show_empty=False is the default in Python 3.13 and ast.dump previously behaved like show_empty=True, the test_ast_expr test currently fails for Python ≥3.13.

Assuming devtools wants to keep the default behavior of ast.dump, I've split the test_ast_expr test into two parts, one testing for the default behavior under Python <3.13, one testing for the default behavior under Python ≥3.13.

See also: https://hydra.nixos.org/build/294880588/nixlog/2/tail