Fix compatibility of check_copy_ast_without_context with Py 3.13b1

[frenzymadness]

Resolves: https://github.com/alexmojaki/pure_eval/issues/16

I've tried to split the regex for better readability but feel free to tell me your preference.

[frenzymadness]

An alternative regex might be r"(, |(?<=\())ctx=(Load|Store|Del)\(\)".

[alexmojaki]

Thanks!