bpo-36124: Add PyInterpreterState.dict.

[ericsnowcurrently]

This parallels PyThreadState.dict. Likewise we add PyInterpreterState_GetDict().

https://bugs.python.org/issue36124

[ericsnowcurrently]

FWIW, I'm not convinced we should add this (as I noted on the tracker issue). So I've marked the PR DO-NOT-MERGE until that question has been resolved.

[arigo]

interp->dict leaks when the PyInterpreterState is destroyed. Please Py_CLEAR() it! Also, make sure it is done at the very last moment; otherwise, race conditions can recreate interp->dict after it was cleared, and then that would leak.

[ericsnowcurrently]

@arigo, thanks for noticing that. Something was nagging at me and I couldn't figure out what. You saved me some time. :) That's what I get for hurrying out a PR at the end of the day!

When you are talking about "at the very last moment", you mean after everything has been cleared which might cause interp->dict to get re-created during its __del__()? Isn't that mostly true of everything that gets cleared in PyInterpreterState_Clear()? Regardless, I'm clearing it almost at the end. Only the "at-fork" handlers are later.

[arigo]

Thanks! At least, the cffi tests pass with PyInterpreter_GetDict().