_AssignmentResult has been removed from cocotb main branch

[davekeeshan]

_AssignmentResult was remove from the main branch of cocotb:

remove deprecated handle <= syntax

it is used in bus.py in the _le_ overload cases, unsure why that was done here, I am assuming it was an upstream problem in cocotb and can be wholly removed

[davekeeshan]

I see this has been brought up in #64