[mlir] Add Index Type, Floating Point Type and None Type subclasses to python bindings.

Based on the PyType and PyConcreteType classes, this patch implements the bindings of Index Type, Floating Point Type and None Type subclasses.
These three subclasses share the same binding strategy:
- The function pointer `isaFunction` points to `mlirTypeIsA***`.
- The `mlir***TypeGet` C API is bound with the `***Type` constructor in the python side.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D86466