Runtime type -> _type, isPython -> isPython, pySlice -> _pySlice to prevent conflicts.

Noticed that now that we actually have Python dictionaries interpreting themselves as dictionaries more often, they conflict with any dictionary that should have a `type` property. So I renamed the internal use property to `_type`. While at it, I also did `_isPython` and `_pySlice`, just to be on the safe side, to make it clear that those are internal bits of the Python runtime separate from user code.