Make it possible to create DenseElementsAttrs with arbitrary shaped types in Python bindings

Right now the bindings assume that all DenseElementsAttrs correspond to tensor values,
making it impossible to create vector-typed constants. I didn't want to change the API
significantly, so I opted for reusing the current signature of `.get`. Its `type` argument
now accepts both element types (in which case `shape` and `signless` can be specified too),
or a shaped type, which specifies the full type of the created attr (`shape` cannot be specified
in that case).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D145053