[mlir][linalg][transform][python] Add mix-in for BufferizeToAllocOp.

Re-apply https://reviews.llvm.org/D157704.

The original patch broke the tests on Python 3.8 and got reverted by
0c4aad05. This patch replaces the usage
of the vertical bar operator for type hints with `Union`.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D158075