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

[nwinter]

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.

[differentmatt]

Thanks for fixing these poorly named internal properties!