Change new_proposal.py to be upstream-agnostic

[jonmeow]

There are issues with specifying push -u origin in new_proposal.py. gh can result in a repo either using upstream+origin, or origin+fork, with the second name being the fork in both cases. Either we need to use gh pr create alone (which may be ideal) or change the git push to be agnostic about the name (likely having to detect which to use, or at least if origin is incorrect).

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