rust-cpython: make PySharedRef::try_borrow_mut() return BorrowMutError...
rust-cpython: make PySharedRef::try_borrow_mut() return BorrowMutError
As I said, it shouldn't be an error of Python layer, but is something like
a coding error. Returning BorrowMutError makes more sense.
There's a weird hack to propagate the borrow-by-leaked state to RefCell
to obtain BorrowMutError. If we don't like it, maybe we can add our own
BorrowMutError.
