[mlir][python] Swap shape and element_type order for MemRefType.

* Matches how all of the other shaped types are declared.
* No super principled reason fro this ordering beyond that it makes the one that was different be like the rest.
* Also matches ordering of things like ndarray, et al.

Reviewed By: ftynse, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D94812