gh-84583: Make pdb enter post-mortem mode even for SyntaxError

[gaogaotiantian]

In #60384, a special check for SyntaxError is introduced in pdb's main() function to avoid infinite loop in pdb if the source file has a SyntaxError. The solution is not elegant enough as we could have a real SyntaxError which we want to debug at run-time. #84583 gave an example with ast.literal_eval(''). This could happen in other scenarios as well, when we compile dynamically generated code.

The original check for SyntaxError was just to prevent pdb from stuck, we can easily do that by moving the pdb._user_requested_quit outside of the try statement, so the quit request from post-mortem mode can be respected too.

This has one minor behavior change - when the user tries to "exit" from the post mortem debugging, for example, using Ctrl+D or quit command, pdb used to restart the program, now it will exit.

I think this is a more reasonable behavior - we told the users explicitly to use cont and step to restart the program, and exit should just mean "exit".

• Issue: pdb does not drop into debugger upon SyntaxError caused by ast.literal_eval #84583

[iritkatriel]

Thanks.