Can't exit from ptpython 3.0.18

[yatmanov]

@jonathanslenders, thank you for the great tool!

Few days ago after I upgrade to ptpython version 3.0.18 I found out I can no longer exit from repl using either exit() or raise SystemExit.
I think the changes caused the issue is in commit 321d9e1. There on line 101 should be raise instead on return. Changing this line to raise fixes #463 too.
I can try to work on the fix if you don't mind.

[jonathanslenders]

Possible fix: prompt-toolkit/python-prompt-toolkit#1457
Would be great if you could try it out, if you have some time.

[jonathanslenders]

Good point, it's indeed line 101! I'll fix it here in ptpython. But the "fix" in prompt_toolkit probably doesn't hurt either.

[yatmanov]

@jonathanslenders, 3.0.19 works well now. I'll close this issue. Thank you!

I'm not sure, but I think we should change return to raise in the run_and_show_expression_async function too.