Add an empty tuple as `__slots__` to `istr`

[jonathandung]

Because istr is a str subclass and strings have no instance dictionaries, and especially in the case of this library where instances of istr are used as dictionary keys and many of them are created for large case-insensitive multidicts, it would be beneficial to have a __slots__ = () declaration in the class body of istr. This will save the memory allocated to create a dictionary every time a istr is constructed to be used as a key.

[Vizonex]

Because istr is a str subclass and strings have no instance dictionaries, and especially in the case of this library where instances of istr are used as dictionary keys and many of them are created for large case-insensitive multidicts, it would be beneficial to have a __slots__ = () declaration in the class body of istr. This will save the memory allocated to create a dictionary every time a istr is constructed to be used as a key.

@jonathandung
While you could do that, one of the issues is that istr has editable attributes. __slots__ however can eliminate some overhang, the problem is that istr has editable attributes which I have explained in your recent PR that you have made.

Solution: You can define these attributes in the __slots__ object and from there the performance enhancements you are looking for can be safely achieved.