Dedenting copied code for checkpoint_paste

[negvoss]

For some reason, it seems like this may only cause an issue for certain users (potentially Windows users in particular).

[YusufErdemK]

Nice fix! The rstrip before dedent is key — textwrap.dedent bails if lines have inconsistent trailing whitespace, which Windows clipboard is notorious for adding. Makes sense this was Windows-specific.
One minor thing: could simplify to a one-liner if you want:

pythoncode_string = textwrap.dedent("\n".join(line.rstrip() for line in code_string.splitlines()))

But honestly the current version reads fine too. Either way, LGTM.