Allow creation of pull requests between sibling repos (forks of same parent) (BB-7166)

I recently started working on the PEP 432 implementation as a branch in my cpython_sandbox repo (https://bitbucket.org/ncoghlan/cpython_sandbox/commits/featurebranches)

After asking for help with patches to keep the Windows builds working, Richard Oudkerk discovered that it isn't possible for him to send a pull request from his existing clone of the CPython mirror to my sandbox, which is also a clone of the CPython mirror.

This seems like an unnecessary restriction, since I was still able to do the merge manually by pulling the change from Richard's clone and merging it into mine.